Shape Controllable Preparation of PbS Crystals by a Simple Aqueous Phase Route Ni,†
Liu,†
Yonghong Hongjiang Xiang Ma,‡ and Zheng Xu*,†
Fei
Wang,†
Yongye
Liang,†
Jianming
Hong,‡
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 4 759-764
State Key Laboratory of Coordination Chemistry, and Center of Material Analysis, Nanjing University, Nanjing, 210093, People’s Republic of China Received June 17, 2003
ABSTRACT: In the present paper, we successfully prepared PbS microcrystals with a flower-shaped structure in a simple aqueous system using microwave irradiation and systematically researched various factors affecting the growth of the flower-shaped PbS crystals. Experimental results indicated that the change of the molar ratio of Pb2+/ S2O32- could significantly influence the morphology of the product while keeping the other experimental conditions constant: with the decrease of the molar ratio of Pb2+/S2O32- from 1:1 to 1:4, the shapes of the products varied from rod to cuboid to flower. Changing the irradiation power in a small range (10-40%) had little effect on the shape of the product. When thiourea and thioacetamide were used as sulfur ion sources instead of Na2S2O3, the shapes of the as-obtained PbS crystals were dendritic and platelike, respectively. The above results further confirmed that Na2S2O3 played an important role in the formation process of PbS crystals with the flower-shaped structure. At the same time, transmission electron spectroscopy observations showed that the counterions for Pb2+ could influence the shape of the product: when Pb(NO3)2 and PbCl2 were used as the lead ion sources, cuboid PbS crystals were produced; while when PbSO4 was used as the lead ion source, the flower-shaped PbS crystals were obtained. A growth mechanism of the flower-shaped PbS crystals was proposed. 1. Introduction Binary II-VI semiconductor materials are useful in various devices including light-emitting diodes,1 single electron transistors,2 and field effect thin film transistors,3 due to their outstanding electronic and optical properties. In principle, the electronic and optical properties of semiconductor materials can be tunable by varying their shapes and sizes,4 so it is one of the desired goals in materials science to realize precise control of the morphology of semiconductor materials. Lead sulfide (PbS) is an important, direct band gap semiconductor material with a rather small bulk band gap (0.41 eV at 300 K) and a larger excitation Bohr radius of 18 nm,5 and it has been widely used in many fields such as in Pb2+ ion selective sensors,6 photography,7 IR detectors,8 and solar absorbers.9 Moreover, an exceptional third-order nonlinear optical property of PbS nanoparticles has been found, which may be useful in optical devices such as an optical switch.10 Also, it is predicted that as compared with GaAs or CdS with a given particle size, the nonlinear properties of PbS will be best.11 Conventionally, PbS was precipitated via the reaction between a dissoluble lead salt and H2S gas in aqueous media. With the progress of materials science, PbS has been prepared in polymers,12,13 zeolites,14 block copolymer nanoreactors,15 inverse micelles,16 microemulsions,17 and the ethanol system through γ-irradiation.18 However, most of the above methods produced nearspherical PbS particles. Recently, rectangular and rodlike PbS nanocrystals have been successfully prepared * To whom correspondence should be addressed. E-mail: zhengxu@ netra.nju.edu.cn. † State Key Laboratory of Coordination Chemistry. ‡ Center of Material Analysis.
in the system containing the organic polyamines with N-chelation properties such as triethylenetetramine19 and ethylenediamine.20 However, systematic research on shape controllable preparation of PbS in a simple aqueous solution is still not found in the literature. Recently, we became interested in searching for a facile method to prepare shape controllable PbS particles via a microwave irradiation technique. Employing (CH3COO)2Pb and Na2S2O3/thiourea as the lead and sulfur sources without the assistance of any surfactant or template, we obtained some very interesting results.21,22 In this work, we report the formation of PbS microcrystals with a flower-shaped structure in a simple aqueous system under microwave irradiation and systematically researched various factors affecting the growth of the flower-shaped PbS crystals including the molar ratio of Pb2+/S2O32-, the lead and sulfur ion sources, and the time and power of microwave irradiation. 2. Experimental Section In a typical experiment, all compounds were analytically pure and used without further purification. A series of aqueous solutions of (CH3COO)2Pb and Na2S2O3 were prepared in the desired mole ratio of Pb2+/S2O32- (Table 1). The solutions were heated in a microwave oven (650 W, 2.45 MHz) for a given time and power. The black precipitates were collected, washed with distilled water several times, and dried in air at 60 °C. X-ray powder diffraction (XRD) patterns of the products were recorded on a Japan Rigaku D/max γA X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 0.154178 nm), using a scanning rate of 0.02 deg/s in 2θ ranges from 20 to 70°. The transmission electron spectroscopy (TEM) images of the products were taken on a JEM200CX, JEOL Transmission Electron Microscope, employing an accelerating voltage of 200 kV. Scanning electron micros-
10.1021/cg034103f CCC: $27.50 © 2004 American Chemical Society Published on Web 05/21/2004
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Table 1. Experimental Conditions and the Shapes of the Products no.
(CH3COO)2Pb (mol)
Na2S2O3 (mol)
power of microwave irradiation (W)
time of microwave irradiation (min)
shapes of particles
1 2 3 4 5 6 7 8 9 10 11
0.0025 0.0025 0.0025 0.0025 0.0025 0.0025 0.0025 0.0025 0.0025 0.0025 0.0025
0.0025 0.005 0.0075 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100
130 130 130 130 65 195 260 130 130 130 130
30 30 30 30 30 30 30 5 10 15 20
rod (main) + cuboid cuboid (main), rod, flower flower (starting to grow) flower (perfect) flower (unperfect) flower (unperfect) flower (unperfect) cuboid cuboid flower (starting to grow) flower (grown-up)
copy (SEM) images were obtained on a scanning electron microscope, Hitachi, ×650/EDAX, PV9100.
3. Results and Discussion 3.1. Formation Process of PbS Crystals. In this work, the system only contained three components: (CH3COO)2Pb, Na2S2O3, and H2O. No surfactant was used. Because Pb2+ ions can form complexes with excess S2O32- ions, the system was a clean and transparent solution before microwave heating. When the system was heated by microwave, Na2S2O3 hydrolyzed to free S2- ions, which reacted with Pb2+ ions to produce PbS. To ascertain the hydrolysis product of Na2S2O3, 1 mL of dilute mixed solutions of Ba(NO3)2 and HNO3 was dropped into the system after PbS had been filtrated off. A white precipitate was produced, which indicated that SO42- ions were one of the hydrolysis products of Na2S2O3. The possible reactions are listed as follows:
S2O32- + H2O f SO42- + H2S Pb2+ + H2S f PbS + 2H+ To explore the shape evolution of PbS crystals with the irradiation time, we prepared a series of products under the same molar ratio (Pb2+/S2O32- ) 1:4) and irradiation power (20%, 130 W) for 5, 10, 15, 20, and 30 min, respectively. The morphologies of the as-prepared products were studied by TEM/SEM techniques. Figure 1 shows TEM images of the products fabricated at various times. When the system was heated for 5 min, the cuboid product was obtained (Figure 1A is a top view and Figure 1B is a high magnification view after rotating the copper grid 45°). According to Figure 1A, the sizes of the particles are not uniform, but most of them range from 20 nm × 20 nm to 30 nm × 40 nm. After 10 min, the size of the cuboid particles increases to ca. 250 nm × 250 nm (Figure 1C). Simultaneously, some rods composed of the cuboids via self-assembly can also be found (Figure 1D). Some flowerlike products start to grow after 15 min and completely form after 20 min (Figure 1E,F). By prolonging the heating time to 30 min, the more perfect crystal with the flower-shaped structure is obtained (Figure 1G). The size of the particles also increases to the micrometer level. Figure 2 depicts SEM images of the products prepared after heating for 15, 20, and 30 min, respectively, which clearly show the evolution of the flower-shaped PbS crystals. It has been found by Murphy that the preferential adsorption of molecules and ions in solution to different
Figure 1. TEM images of the products prepared at various times: (A,B) 5, (C,D) 10, (E) 15, (F) 20, and (G) 30 min.
crystal faces directs the growth of nanoparticles into various shapes by controlling the growth rates along different crystal axes.23 For a face-center-cubic (fcc) nanocrystal, Wang suggested that the shape was mainly determined by the ratio of the growth rate in the 〈100〉 to that in the 〈111〉, and cubes bound by the six {100} planes will be formed when the ratio is relatively
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Figure 3. TEM images of the as-obtained PbS crystals with flower-shaped structures: (a) four petals, (b) six petals, (c) eight petals, and (d) SAED pattern of the sample shown in panel a.
Figure 2. SEM images of the products prepared after different heating times: (A,D) 15, (B) 20, and (C) 30 min.
lower.24 Cheon and co-workers verified the above conclusion. They found that the faster growth on the {111} faces favors the formation of cube-shaped PbS crystals.25 Recently, Qian et al. further confirmed the above conclusion during research on the shape evolution of fcc Cu2O crystals. They observed the shape evolution of Cu2O crystals ranging from eight pod particles through star-shaped particles to cubes.26 In our experiments, we observed an inverse shape evolution of PbS crystals on the basis of the TEM and SEM analyses: namely, from cuboid particles to eight pod particles. Nevertheless, we consider that the shape evolution of PbS crystals can be explained by the above conclusion. At the initial stage, because of the production of PbS, the supersaturation of the system rapidly increased and the produced PbS gradually nucleated and grew. Because of the faster growth on the {111} faces, the cuboid products were obtained (Figure 1A-D). Then, the adsorption and deadsorption of the excess thiosulfate ions on the different planes of cuboid PbS particles may kinetically favor the preferential crystal growth along eight 〈111〉 directions. As a result, continuous growth of PbS in the 〈111〉 direction leads to the formation of flower-shaped PbS (eight pod) crystals. SEM observations confirmed the shape evolution of PbS crystals from cuboid to flowerlike. In Figure 2A,D, some cubes and eight pod structures coexist in the products. In particular, an intermediate with a concave center in every plane of cuboid PbS crystals is clearly shown in Figure 2D (arrow direction). By prolonging the reaction time, the flower-shaped PbS crystals were obtained (see Figure 2B,C).
Figure 4. XRD patterns of PbS crystals prepared in this simple system employing various molar ratios of Pb2+/S2O32(1:1, 1:2, 1:3, and 1:4).
Figure 3 depicts various shapes of the products with four, six, and eight pods, which originate from the different orientation of the same flower-shaped structure with eight pods or overlap of two flower-shaped structures with eight pods. The flower-shaped structures with eight pods take a random orientation on a carbon-covered copper grid. Because it is the steadiest state that any plane of the cube is parallel to that of the copper grid, TEM observation shows that four pod flowers were the main morphology, and while one 3-fold axis of the cube (e.g., A location in Figure 2D) is perpendicular to the plane of copper grid, six pods are present in TEM image. When two flower-shaped crystals happen to stack together with 45° rotations, then eight pods can be seen in the TEM image. Furthermore, SAED patterns of the four pods of the sample shown in Figure 3a are completely identical, which indicate that the flower-shaped particle is a single crystallite and can be indexed as cubic PbS (Figure 3d). 3.2. Influence of the Amount of Na2S2O3. It is wellknown that S2O32- ions have a strong coordinating ability and can form a complex with Pb2+ ions in aqueous solution. Because S2O32- ions are used as both sulfur source and coordinating reagent in the system,
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Figure 5. TEM and SEM images of the products prepared employing different molar ratios of Pb2+/S2O32-: (A,D) 1:1, (B,E) 1:2, (C,F) 1:3, and (G) 1:4.
the amount of S2O32- ions used can affect the stability of the complex and the formation rate of PbS with a certain shape. Figure 4 shows the XRD patterns of samples prepared in this simple system with various molar ratios of Pb2+/S2O32- (1:1, 1:2, 1:3, and 1:4) at the power of 20% (130 W) for 30 min. Although the molar ratios of Pb2+/S2O32- are different, the products prepared have good crystallinity and similar size due to the same self-high-widths. However, the observations from TEM and SEM show that the shapes of the products vary from rodlike, cuboid to flowerlike with the decrease of the molar ratios of Pb2+/S2O32-. Figure 5 depicts TEM and SEM images of the products prepared with different molar ratios of Pb2+/S2O32-. When the molar ratio of Pb2+/S2O32- is 1:1, the main shape of the product is rod (Figure 5A,D). Decreasing the ratio to 1:2, the cuboid product is formed and a few flowers start to grow concomitantly (Figure 5B,E). When the ratio is reduced to 1:3, a great number of flowers can be found (Figure 5C,F); further decreasing the ratio to 1:4, many perfect crystals with the flower-shaped structure are produced (Figure 1G and Figure 5G). These facts indicate that the amount of S2O32- ions can influence the morphology of the product indeed. 3.3. Influence of the Power of Microwave Irradiation. Because the high power of microwave irradiation can make the system boil rapidly, which accelerates the hydrolysis of S2O32- ions, the product can be formed quickly. In general, however, a fast reaction rate will lead to the formation of a crystal with
defects. To ascertain the influence of the power of microwave irradiation on the shape of the products, we prepared a series of products at powers of 10, 20, 30, and 40%, respectively (Table 1) under the same reaction time (30 min) and molar ratio of Pb2+/S2O32- (1:4). Figure 6 depicts TEM images of products prepared at powers of 10, 30, and 40% for 30 min. The flower-shaped products were formed in these cases, indicating that the change of the power in a small range has little influence on the shape of PbS. 3.4. Influence of Sulfur and Lead Ion Sources. To explore the influence of different sulfur ion sources on the shape of PbS crystals under the same experimental conditions (the power of 20%, 30 min, and the Pb2+/sulfur source molar ratio of 1:4), we employed thiourea (NH2CSNH2) and thioacetamide (CH3CSNH2) instead of Na2S2O3 as the sulfur ion sources. Figure 7 shows the TEM images of the as-prepared products. When thiourea was used as the sulfur ion source, the shape of the as-obtained PbS crystals was dendritic (Figure 7A); when thioacetamide was used as the sulfur ion source, the shape of the as-obtained PbS crystals was platelike (Figure 7B) under the above conditions. These results also imply that Na2S2O3 played an important role in the formation process of PbS crystals with a flower-shaped structure. Furthermore, we also investigated the influence of different lead ion sources on the morphology of PbS crystals under the same experimental conditions (20% power, 30 min, and the Pb2+/S2O32- molar ratio of 1:4)
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Figure 6. TEM images of the products prepared employing different powers: (A) 10, (B) 30, and (C) 40%. Table 2. Shapes of the Products Prepared by Employing Various Sulfur Ion and Lead Ion Sources no. 1 2 3 4 5
lead ion source
sulfur ion source
power of microwave irradiation (W)
time of microwave irradiation (min)
(CH3COO)2Pb 0.0025 mol (CH3COO)2Pb 0.0025 mol Pb(NO3)2 0.0025 mol PbSO4 0.0025 mol PbCl2 0.0025 mol
thiourea 0.01 mol thioacetamide 0.01 mol Na2S2O3 0.01 mol Na2S2O3 0.01 mol Na2S2O3 0.01 mol
130
30
dendritic
130
30
cuboid and platelike
130
30
cuboid
130
30
flower (perfect)
130
30
cuboid
Figure 7. TEM images of the as-prepared products, keeping the power at 20%, the time at 30 min, and the Pb2+/sulfur source molar ratio at 1:4 constant: (A) thiourea and (B) thioacetamide as sulfur ion sources, respectively.
except that (CH3COO)2Pb was substituted by Pb(NO3)2, PbSO4, and PbCl2, respectively. The results are listed in Table 2. TEM observations showed that cuboid PbS crystals were produced when Pb(NO3)2 and PbCl2 were used as the lead ion sources, but with PbSO4 as the lead ion source, most of the as-obtained PbS crystals were flower-shaped products with four pods (Figure 8). The above experiments indicate that the counterion in lead salts can affect the shape of the product: The influence
shapes of particles
of NO3- and Cl- ions is similar, but SO42- and CH3COOalso play a similar role. We consider that the above phenomena result from the different interactions between anions and Pb2+ cations. During the growth of PbS crystals, NO3- and Cl- anions have a weak coordination with Pb2+ cations, while SO42- and CH3COOanions have a strong coordination with Pb2+ cations. When Pb(NO3)2 and PbCl2 were used as the lead ion sources, only excess S2O32- ions coordinated with Pb2+ cations. Because of the quicker growth on the {111} faces, the PbS cubes were obtained; when PbSO4 and (CH3COO)2Pb were used as the lead ion sources, a synergic coordination to Pb2+ ions existed between SO42-, CH3COO-, and excess S2O32- anions. The adsorption and deadsorption rates of the excess thiosulfate ions on the different planes of PbS nuclei might be influenced by SO42- or CH3COO- ions, which might favor the preferential crystal growth along eight 〈111〉 directions. Finally, the flower-shaped PbS crystals were formed. 4. Conclusions PbS microcrystals with the flower-shaped structure have been successfully synthesized in a simple aqueous system under the assistance of microwave irradiation, employing (CH3COO)2Pb and Na2S2O3 as the starting
Figure 8. TEM images of the products prepared by employing Pb(NO3)2, PbSO4, and PbCl2 as the lead ion sources instead of (CH3COO)2Pb under the same experimental conditions (20% power, 30 min, and the Pb2+/S2O32- molar ratio of 1:4): (A) Pb(NO3)2, (B) PbSO4, and (C) PbCl2.
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material. The formation process of the flower-shaped PbS crystals was discussed systematically. The research showed that the shapes of the products varied from rodlike, cuboid to flower-shaped with the decrease of the molar ratio of Pb2+/S2O32- from 1:1 to 1:4 and that the optimum irradiation power is 20% (130 W). The sulfur ion and lead ion sources can also affect the morphology of the product. The growth mechanism of the flowershaped PbS crystals was suggested as follows: in the initial stage, the cuboid products were obtained due to the faster growth on the {111} faces. Then, the adsorption and deadsorption of the excess thiosulfate ions on the different planes of cuboid PbS particles might kinetically favor the preferential crystal growth along eight 〈111〉 directions, which led to the formation of the flower-shaped PbS crystals. Acknowledgment. We thank the Natural Science Foundation of Jiangsu Province for fund support (Grant BK2002076). References (1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (3) Ridley, B. A.; Nivi, B.; Jacobson, J. M. Science 1999, 286, 746. (4) Yang, P. D.; Lieber, C. M. Science 1996, 273, 1836. (5) Machol, J. L.; Wise, F. W.; Patel, R. C.; Tanner, D. B. Phys. Rev. B 1993, 48, 2819. (6) Hirata, H.; Higashiyama, K. Bull. Chem. Soc. Jpn. 1971, 44, 2420.
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