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doi:10.1016/j.jcr

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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 yieldsvia a solvothermal process

Liqiang Xua,b, Wanqun Zhangb, Yanwei Dinga, Weichao Yub, Jinyun Xinga,Fanqing Lia, Yitai Qiana,b,�

aStructure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P.R. ChinabDepartment 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 significantconcerns in the fabrication of semiconductors,metal nanocrystals and other inorganic materials,because they can determine the unique chemical and

e front matter r 2004 Published by Elsevier B.V.

ysgro.2004.08.024

ng author. Structure Research Laboratory,

cience and Technology of China, Hefer, Anhui

hina Tel.: +86-551-360 2942; fax: +86-551-

ss: ulq@mail.ustc.edu.cn (Y. Qian).

physical properties of the materials [1–3]. Althoughnumerous examples have been reported, shape andsize have been difficult to control and will be a greatchallenge for the future. Therefore, it is technolo-gically important to understand the growth historyand the shape-guiding process of crystals, so that itwill be possible to program the system to yieldcrystals with a desired shape and/or size [4].

As an important II–VI semiconductor, leadsulfide (PbS) has attracted considerable attentionfor many decades. Interest in PbS arises due to itssmall band gap energy (0.41 eV) and large exciton

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L. Xu et al. / Journal of Crystal Growth 273 (2004) 213–219214

Bohr radius (18 nm) at room temperature, whichpermit size-quantization effects to be clearly visibleeven for relatively large particles or crystallites [5],and make it an interesting system for studying theeffects of size confinement. PbS is potentiallyuseful for making devices that require small bandgap semiconductors with optical absorption andemission in the red and near-infrared region of thespectrum. In addition, the exceptional third-ordernon-linear optical properties of PbS nanoparticlesalso 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 g-irradiated non-aqueous solution [13]. Recently,rectangular and rod-like PbS crystals have beensuccessfully prepared in biphasic solvothermalinterface reactions route and in systems containingorganic polyamines with N-chelating propertiessuch as in triethylenetetramine [14,15]. Closed PbSnanowires are achieved using a solvothermalmethod in the presence of poly [N-(2-aminoethyl)acrylamide] [16]. Very recently, PbS dendrites havebeen synthesized via a hydrothermal or solvother-mal method [17,18]. Cheon and co-workers havesynthesized PbS nanocrystals with various rod-based structures including highly faceted starshapes, truncated octahedrons and cubes [19].The cubic particles have large surface-to-volumeratios and may prove to have significantly differentreactivity and selectivity in catalysis. Dendritesalso have attracted much attention in recent yearsdue to their interesting morphology and potentialapplications. In this report, PbS microcrystalsincluding multipods, cubes, truncated octahedrons,dendrites and flowers were produced throughadjusting the reactants, solvents, and duration timeof the reaction at 180 1C. Several influential factorson the shape evolutions of the PbS microcrystalswere investigated. Based on the observed shapeevolutions, a possible formation mechanism wasproposed for the formation of PbS cubes.

2. Experimental section

All regents were of analytical grade andpurchased from Shanghai Chemistry Co. without

further purification. In a typical procedure, 1.0 g ofPbCl2 and 1.19 g of CH5N3S were used asreactants to synthesize PbS cubes. The reactantswere loaded into a 60-ml Teflon-lined stainless-steel autoclave, then filled with diethy leneglycol(DEG) up to 90% of the total volume. Thesolution was agitated until the CH5N3S wasdissolved. After that, the autoclave was sealedand maintained at 180 1C for 12–27 h withoutshaking or stirring during the heating period andthen cooled to room temperature naturally. Theprecipitates were filtered off and washed withabsolute ethanol. After drying in a vacuum at50 1C for 4 h, the final products were collected forcharacterization. In order to investigate the para-meters influencing the morphologies of PbSmicrocrystals, a series of experiments were carriedout by changing the reactants and/or solvents at180 1C. The detailed reaction conditions and thecorresponding results are shown in Table 1.

The phase identification of the products wasperformed by an X-ray powder diffraction (XRD)technique using a MAX 18 AHF X-ray diffract-meter (MAC Science Co. Ltd) with Cu Ka1

radiation (l ¼ 1:5418 (A). X-ray photoelectronspectroscopy (XPS) measurements were performedon a VGESCALAB MKII X-ray photoelectronspectrometer with a Mg Ka ¼ 1253:6 eV excitationsource. The morphology and structure of theproducts were examined by a scanning electronmicroscope (SEM) using an X-650 microanalyzer,field emission scanning electron microscopy(FSEM, JEOL JSM-6300F) and transmissionelectron microscopy (TEM, Hitachi H-800 withan 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 directinformation about the structures, sizes and typicalmorphologies of the as-obtained PbS particlesgrown for different periods of time and withdifferent ratios of reactants (for truncated octahe-drons). Fig. 1a shows a typical SEM image ofSample 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 Starting reagents Solvents Time(h) Products

1 PbCl2 (1 g)+CH5N3S (1.19 g)b1 b1DEG 12–24 Multipods and cubes

2 PbCl2 (1 g)+CH5N3S (1.19 g) DEG 28 Cubes

3 PbCl2 (2 g)+CH5N3S (1.19 g) DEG 27 Truncated octahedrons and Cubes

4 PbCl2 (2 g)+CH5N3S (1.19 g) DEG 36 Cubes

5 (1 g)+CH5N3S (1.19 g) DEG+b2EG 12 Dendrites

6 aPb(AC)2 (1 g)+CH5N3S (1.19 g) b3TEG 12 Flowers

7 PbCl2 (1 g)+CS(NH2)2 (1.19 g) (1.19 g) TEG 12 Dendrites

8 Pb(NO3)2 (1 g)+CH5N3S) (1.19 g) TEG 12 Flower

b1DEG: diethylene glycol; b2EG: ethylene glycol; b3TEG: Tetraethylene glycol.aPb(AC)2:Pb(CH3COO)2

L. Xu et al. / Journal of Crystal Growth 273 (2004) 213–219 215

indicating that it is mainly composed of PbS cubesand multipods (�40%). Fig. 1b and c presenthigh-magnification SEM images of three-dimen-sional PbS with six pods and with edge lengths inthe range of 2–3 mm. With increasing durationtime, the as-obtained product is composed ofthree-dimensional PbS eight-pods (�20%) andcubes as can be seen from Fig. 1d and e. Whenthe reaction time was prolonged to 28 h, theproduct 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 imagesof the as-obtained PbS cubes (Sample 2). Theyclearly show that the as-produced PbS possesses acubic structure. These perfect cubes have slicksurfaces, which may be an important feature forconnecting them as building blocks into devices. Aselected area electron diffraction (SAED) patternrecorded from one cube is shown in Fig. 2h, whichreveals its single-crystal nature and the preferredgrowth direction of PbS cubes along [0 0 1] zoneaxis. Numerous SAED pattern analyses demon-strate that the as-formed PbS cubes are singlecrystals. Figs. 1j and k show slick truncated PbSoctahedrons (�40%) coexisting with PbS cubesobtained by using PbCl2 (2.0 g) and CH5N3S(1.19 g) with DEG as the solvent at 180 1C for27 h (Sample 3). It is worth noting that some PbSparticles with six pods coexist with the truncatedoctahedrons, and it is thought that they might beintermediates produced before the formation ofthe truncated octahedrons (as arrowed in Fig. 1k).

It is also found that when the reaction time wasprolonged to 36 h, a large yield of PbS cubes(�95%) was produced, while PbS truncatedoctahedrons were barely observed (Sample 4).

An XRD pattern of the as-obtained PbS cubes(Sample 2, Fig. 1f) is shown in Fig. 2. All thereflection peaks could be indexed to the face-centered cubic (fcc) phase of PbS. After re thecalculated lattice constant a ¼ 5:929 (A) is close tothe value shown in the literature (JCPDS no. 5-592). No obvious impurity phases could bedetected. The narrow diffraction peaks suggestthat the cubes are highly crystalline.

Further information for the elemental composi-tion and oxidation of the surface of the PbS cubeswas obtained using XPS analyses. Fig. 3 shows theXPS spectra of the as-obtained PbS cubes (Sample2). The spectra confirmed the formation of PbScrystals with molar ratios of Pb:S of 1.00:1.09, andno impurity peaks were detected. The surveyspectrum, Pb 4f core level and S2p core levelspectrum are shown in Figs. 3a–c, which give thebinding energy of Pb 4f at 137.0 and 141.9 eV, andof S2p at 160.2 eV, respectively. These results areconsistent with the reported values [20].

4. Discussion

The possible chemical process for the formationof 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).

L. Xu et al. / Journal of Crystal Growth 273 (2004) 213–219216

compounds at 180 1C [21]: secondly, the as-formedcompounds in the hot DEG solutions decomposedto form stable PbS microcrystals after a prolongedduration time. The overall reactions involved inthis experiment can be described as below:

PbCl2 þ CH5N3S ¼ PbS þ CH5N3Cl2:

Prior to this report, Wang [22] suggested thatthe 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 cubesbounded by the six {1 0 0} planes will be formedwhen the ratio is relatively low. The shapeevolution of PbS observed in this experimentprovides a good example of such processes. Thecross-linked six-pods of the PbS particles, whichwere obtained when the reaction time was fixed at12 h (Sample 1), exactly match the six /1 0 0S

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L. Xu et al. / Journal of Crystal Growth 273 (2004) 213–219 217

directions of the cubic lattice of PbS (Figs. 1a–c),indicating a higher growth rate in the /1 0 0S thanin the /1 1 1S direction. When the reaction timewas further prolonged, PbS eight-pods and cubes(shown in Figs. 1d and e) were producedsimultaneously. It is likely that the eight-podsapproximately match the /1 1 1S directions of thecubic lattice of PbS (see Figs. 1d and e), whichreveals that the growth rate in the /1 1 1Sdirections is faster than that of the /1 0 0S

0 200 600 8000

(a)

Pb4d O1s

S2P

(c)

S 2p3

Inte

nsit

y (c

ps)

80000

70000

10000

20000

30000

40000

50000

60000

400 1000Binding Energy (eV)

C(Aug)

Pb 4f

Pb5d

C1S

Pb4p

Inte

nsit

y (c

ps)

3400

3200

3000

2800

2600

2400

Bind

156 158 160

Fig. 3. XPS spectra of the as-obtained PbS cubes. (a) surv

30 40 50 600

420331

111

2θ/degrees

Inte

nsit

y (

a.u.

)

10000

8000

6000

4000

2000

20 8070

200

220

311

222400 422

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

directions during this period. It is found that thefinal products were mainly composed of PbS cubeswhen the duration time was extended to 28 h(Sample 2). The shape evolution of the PbS fromeight-pods to cubes implies that the {1 1 1} facetsof cubic PbS were diminished because of their highgrowth rate, and the {1 0 0} facets remained due totheir lower growth rate. The open spaces betweenthe eight-pods are filled, and the particles withmultipods gradually lost their shape. Finally, themore thermodynamically stable cubes enclosedwith six {1 0 0} planes resulted with increasingreaction time (Figs. 1f, and g. It is reasonable tothink that the formation mechanism of the PbStruncated octahedrons (coexisted with PbS cubes,Sample 3) is similar to that of the PbS cubes. Thatis, they grow more quickly along the /1 0 0Sdirection than that of the /1 1 1S at first (asevidenced by Fig. 1g, and some of them maydevelop into the more thermodynamically stableoctahedron shape with marginal truncation attheir corners (i.e., tetradecahedron) [19]. Withincreasing reaction time and subsequent fastergrowth rate in the /1 1 1S than in the /1 0 0Sdirection, they finally developed into PbS cubes(�95%) (Sample 4).

1000

2000

3000

4000

5000

6000

7000

8000

168 170 172

/2

Inte

nsit

y (c

ps)

132 134 136Binding Energy (eV)

138 140 142 144 146 148(b)

Pb4f 7/2

Pb4f 5/2

ing Energy (eV)

162 164 166

ey spectrum; (b) Pb 4f core level; (c) S2p core level.

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L. Xu et al. / Journal of Crystal Growth 273 (2004) 213–219218

Our systematic studies of varying growth para-meters demonstrated that they are critical factorsfor determining the architectural features of thePbS microcrystals. Firstly, only irregular shapedPbS crystals were produced if the reaction

Fig. 4. SEM images of the as-obtained PbS dendrites and flowers pro

12 h. (a) PbCl2 and CH5N3S with DEG as the solvent. (b,c) PbCl2 a

Pb(AC)2 and CH5N3S with TEG as the solvent. (f, g) Pb(NO3)2 and

temperature was lower than 160 1C. This phenom-ena might be related to the melting point of theCH5N3S (179–182 1C); however, the exact reasonis not clear. Secondly, through changing thereactants or/and solvent, large yields of PbS

duced by using different reactants or/and solvents at 180 1C for

nd CH5N3S with TEG and DEG mixture as the solvent. (d,e)

CH5N3S with TEG as the solvent.

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L. Xu et al. / Journal of Crystal Growth 273 (2004) 213–219 219

flowers and dendrites were produced insteadof the large yield of PbS cubes (Table 1). Forexample, when DEG was replaced by a mixture ofDEG and EG, PbS dendrites with a yield of �95%were produced (Fig. 4a, Sample 5). IfPb(CH3COO)2 and CH5N3S were used as reac-tants and tetraethylene glycol (TEG) as thesolvent, the sample was mainly composed of PbSflowers with several petals such as with four,six and eight petals (Figs. 4b and c, Sample 6). PbSdendrites with four or six branches shown inFigs. 4d and e (corresponding to Sample 7) wereobtained when PbCl2 and CS(NH2)2 were usedas reactants and TEG used as the solvent.The nanorods grown from each branch areparallel to each other and in the same plane, andthey are perpendicular to the branch. PbSflowers with multiple petals were also producedby employing Pb(NO3)2 and CH5N3S as reactantsand TEG as the solvent. Their SEM imagesare shown in Figs. 4g and f, respectively (Sample8). It is thought that the PbS dendrites andflowers were formed via a nucleation andaggregation process (through oligomericchains) as was suggested by Fenske and co-workers [18].

5. Conclusion

In summary, a convenient solvothermal syn-thetic route has been successfully developed toprepare PbS micro-crystals of various shapes,including multipods, cubes, truncated octahe-drons, dendrites and flowers. It was found thatthe reaction temperatures, reactants, solvents andduration times play important roles in the forma-tion and shapes of the products. A possiblemechanism for the formation of single-crystallinePbS cubes was proposed based on their shapeevolutions. We believe that this method can alsobe extended to the direct growth of otherimportant semiconductor materials with diverseuseful morphologies.

Acknowledgements

Financial support from the National NaturalScience Found of China and the 973 Project ofChina is greatly appreciated.

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