Shape-controlled synthesis of PbS microcrystals in large yields via a solvothermal process

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Journal of Crystal Growth 273 (2004) 213–219 www.elsevier.com/locate/jcrysgro

Shape-controlled synthesis of PbS microcrystals in large yields via a solvothermal process Liqiang Xua,b, Wanqun Zhangb, Yanwei Dinga, Weichao Yub, Jinyun Xinga, Fanqing Lia, Yitai Qiana,b, a

Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China b Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Received 1 January 2004; accepted 12 August 2004 Communicated by R. James

Abstract PbS microcrystals including multipods, truncated octahedrons and cubes were produced by the reaction of PbCl2 and thiosemicarbazide (CH5N3S) using diethylene glycol (DEG) as a solvent at 180 1C. When the reaction time was prolonged, the final products grew into single-crystalline PbS cubes with diameters in the range of 0.3–2 mm. A possible formation mechanism of the PbS cubes was proposed based on their shape evolutions. By changing the reactants or/and solvents, PbS dendrites and flowers could also be produced with high yield. It is found that the reaction temperature, reactants and solvents play important roles on the shape evolutions of the PbS microcrystals. r 2004 Published by Elsevier B.V. Keywords: A1. Dendrites; A1. Multipods; A1. Shape evolution; A1. Truncated octahedrons

1. Instruction Currently, shape and size control are significant concerns in the fabrication of semiconductors, metal nanocrystals and other inorganic materials, because they can determine the unique chemical and Corresponding author. Structure Research Laboratory,

University of Science and Technology of China, Hefer, Anhui 230026, P.R. China Tel.: +86-551-360 2942; fax: +86-5513607402 E-mail address: [email protected] (Y. Qian). 0022-0248/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2004.08.024

physical properties of the materials [1–3]. Although numerous examples have been reported, shape and size have been difficult to control and will be a great challenge for the future. Therefore, it is technologically important to understand the growth history and the shape-guiding process of crystals, so that it will be possible to program the system to yield crystals with a desired shape and/or size [4]. As an important II–VI semiconductor, lead sulfide (PbS) has attracted considerable attention for many decades. Interest in PbS arises due to its small band gap energy (0.41 eV) and large exciton

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Bohr radius (18 nm) at room temperature, which permit size-quantization effects to be clearly visible even for relatively large particles or crystallites [5], and make it an interesting system for studying the effects of size confinement. PbS is potentially useful for making devices that require small band gap semiconductors with optical absorption and emission in the red and near-infrared region of the spectrum. In addition, the exceptional third-order non-linear optical properties of PbS nanoparticles also show potential use in high-speed switching [6]. PbS particles have been prepared in polymers [7,8], zeolites [9], block copolymer nanoreactors [10], inverse micelle [11], microemulsion [12] and in girradiated non-aqueous solution [13]. Recently, rectangular and rod-like PbS crystals have been successfully prepared in biphasic solvothermal interface reactions route and in systems containing organic polyamines with N-chelating properties such as in triethylenetetramine [14,15]. Closed PbS nanowires are achieved using a solvothermal method in the presence of poly [N-(2-aminoethyl) acrylamide] [16]. Very recently, PbS dendrites have been synthesized via a hydrothermal or solvothermal method [17,18]. Cheon and co-workers have synthesized PbS nanocrystals with various rodbased structures including highly faceted star shapes, truncated octahedrons and cubes [19]. The cubic particles have large surface-to-volume ratios and may prove to have significantly different reactivity and selectivity in catalysis. Dendrites also have attracted much attention in recent years due to their interesting morphology and potential applications. In this report, PbS microcrystals including multipods, cubes, truncated octahedrons, dendrites and flowers were produced through adjusting the reactants, solvents, and duration time of the reaction at 180 1C. Several influential factors on the shape evolutions of the PbS microcrystals were investigated. Based on the observed shape evolutions, a possible formation mechanism was proposed for the formation of PbS cubes.

2. Experimental section All regents were of analytical grade and purchased from Shanghai Chemistry Co. without

further purification. In a typical procedure, 1.0 g of PbCl2 and 1.19 g of CH5N3S were used as reactants to synthesize PbS cubes. The reactants were loaded into a 60-ml Teflon-lined stainlesssteel autoclave, then filled with diethy leneglycol (DEG) up to 90% of the total volume. The solution was agitated until the CH5N3S was dissolved. After that, the autoclave was sealed and maintained at 180 1C for 12–27 h without shaking or stirring during the heating period and then cooled to room temperature naturally. The precipitates were filtered off and washed with absolute ethanol. After drying in a vacuum at 50 1C for 4 h, the final products were collected for characterization. In order to investigate the parameters influencing the morphologies of PbS microcrystals, a series of experiments were carried out by changing the reactants and/or solvents at 180 1C. The detailed reaction conditions and the corresponding results are shown in Table 1. The phase identification of the products was performed by an X-ray powder diffraction (XRD) technique using a MAX 18 AHF X-ray diffractmeter (MAC Science Co. Ltd) with Cu Ka1 ( radiation (l ¼ 1:5418 A). X-ray photoelectron spectroscopy (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with a Mg Ka ¼ 1253:6 eV excitation source. The morphology and structure of the products were examined by a scanning electron microscope (SEM) using an X-650 microanalyzer, field emission scanning electron microscopy (FSEM, JEOL JSM-6300F) and transmission electron microscopy (TEM, Hitachi H-800 with an accelerating voltage of 200 kV).

3. Results Typical SEM and TEM images of Samples 1–3 (Table 1) are shown in Fig. 1. They provide direct information about the structures, sizes and typical morphologies of the as-obtained PbS particles grown for different periods of time and with different ratios of reactants (for truncated octahedrons). Fig. 1a shows a typical SEM image of Sample 1 prepared by the reaction of PbCl2 (1.0 g) and CH5N3S (1.19 g) in DEG at 180 1C for 12 h,

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Table 1 Effects of reactants, solvents and reaction times on the growth of PbS microcrystals in thermal organic solutions (180 1C). Sample 1 2 3 4 5 6 7 8 b1

Starting reagents b1

PbCl2 (1 g)+CH5N3S (1.19 g) PbCl2 (1 g)+CH5N3S (1.19 g) PbCl2 (2 g)+CH5N3S (1.19 g) PbCl2 (2 g)+CH5N3S (1.19 g) (1 g)+CH5N3S (1.19 g) a Pb(AC)2 (1 g)+CH5N3S (1.19 g) PbCl2 (1 g)+CS(NH2)2 (1.19 g) (1.19 g) Pb(NO3)2 (1 g)+CH5N3S) (1.19 g) DEG: diethylene glycol; b2EG: ethylene glycol; a Pb(AC)2:Pb(CH3COO)2

Solvents

Time(h)

Products

b1DEG

12–24 28 27 36 12 12 12 12

Multipods and cubes Cubes Truncated octahedrons and Cubes Cubes Dendrites Flowers Dendrites Flower

DEG DEG DEG DEG+b2EG b3 TEG TEG TEG

b3

TEG: Tetraethylene glycol.

indicating that it is mainly composed of PbS cubes and multipods (40%). Fig. 1b and c present high-magnification SEM images of three-dimensional PbS with six pods and with edge lengths in the range of 2–3 mm. With increasing duration time, the as-obtained product is composed of three-dimensional PbS eight-pods (20%) and cubes as can be seen from Fig. 1d and e. When the reaction time was prolonged to 28 h, the product is mainly composed of PbS cubes (95%) with diameters in the range of 0.3–2 mm. Figs. 1f, g, h display the FSEM and TEM images of the as-obtained PbS cubes (Sample 2). They clearly show that the as-produced PbS possesses a cubic structure. These perfect cubes have slick surfaces, which may be an important feature for connecting them as building blocks into devices. A selected area electron diffraction (SAED) pattern recorded from one cube is shown in Fig. 2h, which reveals its single-crystal nature and the preferred growth direction of PbS cubes along [0 0 1] zone axis. Numerous SAED pattern analyses demonstrate that the as-formed PbS cubes are single crystals. Figs. 1j and k show slick truncated PbS octahedrons (40%) coexisting with PbS cubes obtained by using PbCl2 (2.0 g) and CH5N3S (1.19 g) with DEG as the solvent at 180 1C for 27 h (Sample 3). It is worth noting that some PbS particles with six pods coexist with the truncated octahedrons, and it is thought that they might be intermediates produced before the formation of the truncated octahedrons (as arrowed in Fig. 1k).

It is also found that when the reaction time was prolonged to 36 h, a large yield of PbS cubes (95%) was produced, while PbS truncated octahedrons were barely observed (Sample 4). An XRD pattern of the as-obtained PbS cubes (Sample 2, Fig. 1f) is shown in Fig. 2. All the reflection peaks could be indexed to the facecentered cubic (fcc) phase of PbS. After re the calculated lattice constant a ¼ 5:929 (A˚) is close to the value shown in the literature (JCPDS no. 5592). No obvious impurity phases could be detected. The narrow diffraction peaks suggest that the cubes are highly crystalline. Further information for the elemental composition and oxidation of the surface of the PbS cubes was obtained using XPS analyses. Fig. 3 shows the XPS spectra of the as-obtained PbS cubes (Sample 2). The spectra confirmed the formation of PbS crystals with molar ratios of Pb:S of 1.00:1.09, and no impurity peaks were detected. The survey spectrum, Pb 4f core level and S2p core level spectrum are shown in Figs. 3a–c, which give the binding energy of Pb 4f at 137.0 and 141.9 eV, and of S2p at 160.2 eV, respectively. These results are consistent with the reported values [20].

4. Discussion The possible chemical process for the formation of PbS can be expressed as follows: firstly, Pb2+ combines with CH5N3S to form coordination

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Fig. 1. Representative SEM and TEM images of the as-obtained PbS particles obtained for different duration times or with different ratios of reactants. (a–c) for 12 h, (d,e) for 18 h, (f,g) for 27 h. (h) TEM image of the PbS cubes. (i) Typical SAED pattern of one PbS cube. (j) SEM image of the truncated octahedrons of PbS obtained at 180 1C after 27 h. (k) A high magnification SEM image of (j).

compounds at 180 1C [21]: secondly, the as-formed compounds in the hot DEG solutions decomposed to form stable PbS microcrystals after a prolonged duration time. The overall reactions involved in this experiment can be described as below: PbCl2 þ CH5 N3 S ¼ PbS þ CH5 N3 Cl2 : Prior to this report, Wang [22] suggested that the shape of an fcc nanocrystal was mainly

determined by the ratio of the growth rate in the /1 0 0S to that in the /1 1 1S direction, and cubes bounded by the six {1 0 0} planes will be formed when the ratio is relatively low. The shape evolution of PbS observed in this experiment provides a good example of such processes. The cross-linked six-pods of the PbS particles, which were obtained when the reaction time was fixed at 12 h (Sample 1), exactly match the six /1 0 0S

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directions during this period. It is found that the final products were mainly composed of PbS cubes when the duration time was extended to 28 h (Sample 2). The shape evolution of the PbS from eight-pods to cubes implies that the {1 1 1} facets of cubic PbS were diminished because of their high growth rate, and the {1 0 0} facets remained due to their lower growth rate. The open spaces between the eight-pods are filled, and the particles with multipods gradually lost their shape. Finally, the more thermodynamically stable cubes enclosed with six {1 0 0} planes resulted with increasing reaction time (Figs. 1f, and g. It is reasonable to think that the formation mechanism of the PbS truncated octahedrons (coexisted with PbS cubes, Sample 3) is similar to that of the PbS cubes. That is, they grow more quickly along the /1 0 0S direction than that of the /1 1 1S at first (as evidenced by Fig. 1g, and some of them may develop into the more thermodynamically stable octahedron shape with marginal truncation at their corners (i.e., tetradecahedron) [19]. With increasing reaction time and subsequent faster growth rate in the /1 1 1S than in the /1 0 0S direction, they finally developed into PbS cubes (95%) (Sample 4).

directions of the cubic lattice of PbS (Figs. 1a–c), indicating a higher growth rate in the /1 0 0S than in the /1 1 1S direction. When the reaction time was further prolonged, PbS eight-pods and cubes (shown in Figs. 1d and e) were produced simultaneously. It is likely that the eight-pods approximately match the /1 1 1S directions of the cubic lattice of PbS (see Figs. 1d and e), which reveals that the growth rate in the /1 1 1S directions is faster than that of the /1 0 0S

10000

200

111

220

4000

311

2000

222

0 20

30

40

420 422 400 331

50 60 2θ/degrees

70

80

Fig. 2. XRD pattern of the as-obtained PbS cubes.

8000

80000

C(Aug)

70000 60000

C1S S2P

40000 30000 20000 0

Pb4f 7/2

6000

Pb4f 5/2

5000 4000 3000 2000

Pb5d

10000

(a)

Pb4d O1s Pb4p

Pb 4f

50000

7000 Intensity (cps)

Intensity (cps)

1000 0

200

600 800 400 Binding Energy (eV)

1000

(b)

132 134 136 138 140 142 144 146 148 Binding Energy (eV)

3400 S 2p3/2 3200 Intensity (cps)

Intensity (a.u.)

8000 6000

3000 2800 2600 2400 156 158 160 162 164 166 168 170 172

(c)

217

Binding Energy (eV)

Fig. 3. XPS spectra of the as-obtained PbS cubes. (a) survey spectrum; (b) Pb 4f core level; (c) S2p core level.

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Our systematic studies of varying growth parameters demonstrated that they are critical factors for determining the architectural features of the PbS microcrystals. Firstly, only irregular shaped PbS crystals were produced if the reaction

temperature was lower than 160 1C. This phenomena might be related to the melting point of the CH5N3S (179–182 1C); however, the exact reason is not clear. Secondly, through changing the reactants or/and solvent, large yields of PbS

Fig. 4. SEM images of the as-obtained PbS dendrites and flowers produced by using different reactants or/and solvents at 180 1C for 12 h. (a) PbCl2 and CH5N3S with DEG as the solvent. (b,c) PbCl2 and CH5N3S with TEG and DEG mixture as the solvent. (d,e) Pb(AC)2 and CH5N3S with TEG as the solvent. (f, g) Pb(NO3)2 and CH5N3S with TEG as the solvent.

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flowers and dendrites were produced instead of the large yield of PbS cubes (Table 1). For example, when DEG was replaced by a mixture of DEG and EG, PbS dendrites with a yield of 95% were produced (Fig. 4a, Sample 5). If Pb(CH3COO)2 and CH5N3S were used as reactants and tetraethylene glycol (TEG) as the solvent, the sample was mainly composed of PbS flowers with several petals such as with four, six and eight petals (Figs. 4b and c, Sample 6). PbS dendrites with four or six branches shown in Figs. 4d and e (corresponding to Sample 7) were obtained when PbCl2 and CS(NH2)2 were used as reactants and TEG used as the solvent. The nanorods grown from each branch are parallel to each other and in the same plane, and they are perpendicular to the branch. PbS flowers with multiple petals were also produced by employing Pb(NO3)2 and CH5N3S as reactants and TEG as the solvent. Their SEM images are shown in Figs. 4g and f, respectively (Sample 8). It is thought that the PbS dendrites and flowers were formed via a nucleation and aggregation process (through oligomeric chains) as was suggested by Fenske and coworkers [18].

5. Conclusion In summary, a convenient solvothermal synthetic route has been successfully developed to prepare PbS micro-crystals of various shapes, including multipods, cubes, truncated octahedrons, dendrites and flowers. It was found that the reaction temperatures, reactants, solvents and duration times play important roles in the formation and shapes of the products. A possible mechanism for the formation of single-crystalline PbS cubes was proposed based on their shape evolutions. We believe that this method can also be extended to the direct growth of other important semiconductor materials with diverse useful morphologies.

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