Journal of Crystal Growth 231 (2001) 248–251

Hydrothermal preparation of uniform cubic-shapedPbS nanocrystals

Yang Jianga, Yue Wua, Bo Xiea, Shengwen Yuana, Xianming Liub, Yitai Qiana,b,*aDepartment of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China

bStructure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China

Received 12 March 2001; accepted 24 May 2001

Communicated by L.F. Schneemeyer

Abstract

Uniform cubic-shaped PbS nanocrystals have been prepared by a hydrothermal reaction between Pb(Ac)2 � 2H2O andNa2S2O3 in the presence of surfactant of C17H33COOK. The product was characterized through XRD, TEM, andHRTEM. Essentially all the face-centered cubic phase of PbS nanocrystals are well-defined cube with (2 0 0) latticeplanes as surfaces and easy to arrange orderly. r 2001 Published by Elsevier Science B.V.

PACS: 81.07.Bc

Keywords: A1. Crystal morphology; A2. Hydrothermal crystal growth; B1. Nanomaterials; B2. Semiconducting II–VI materials

1. Introduction

In recent years, low-dimensional nanocrystallinesemiconductors have been deemed technologicallyimportant in the design and realization of infor-mation processing and storage nanodevices of thefuture. Among these materials, group II–VIcompounds, including PbS have attracted con-siderable attention. Interest in PbS arises due to itssmall band gap energy (0.41 eV) and large excitonBohr radius (18 nm), which permit size-quantiza-tion effects to be clearly visible even for large

particles or crystallites [1]. In addition, the non-linear optical properties of PbS nanoparticles showthe potential use in high-speed switching [2].Usually, PbS nanoparticles have been studied inmicelles [3], glass [4,5], zeolites [6], sol–gel, [7] andLangmuir–Blodgett film [8], or using single mole-cule precursor approach [9]. Our group hasreported room temperature chemical reactionand g-radiation synthesis routes to nanocrystallinePbS [10,11]. In most of the above cases, thenanoparticles have poorly defined surfaces and abroad distribution of particle sizes. Recently, PbSnanoparticles and nanorods assemblied in multi-layers polymer films have been reported [1,12]. Butthe challenge for the control of sizes, size distribu-tion and morphology of nanoparticles still re-mains. Herein we report a hydrothermal approach

*Corresponding author. Department of Chemistry, Univer-

sity of Science and Technology, Hefei, Anhui 230026, People’s

Republic of China. Fax:+86-551-3631-760.

E-mail address: djy@mail.ustc.edu.cn (Y. Qian).

0022-0248/01/$ - see front matter r 2001 Published by Elsevier Science B.V.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 5 1 0 - X

to uniform cubic-shaped PbS nanocrystals in thepresence of surfactant.

2. Experimental section

Pb(Ac)2 � 2H2O and Na2S2O3 with analyticgrade purity were used as starting materials, andC17H33COOK as surfactant. Three reagents weredissolved in distilled water to prepare aqueoussolutions with a concentration of 0.01M, res-pectively. In a typical procedure, 15mlPb(Ac)2 � 2H2O solution and 15ml Na2S2O3 solu-tion were put into a Teflon-lined stainless steelautoclave, then 20ml surfactant solution wasadded to fill the autoclave up to 90% of the totalvolume. The sealed autoclave was maintained at1801C for 24 h, then cooled to room temperaturenaturally. A gray-black precipitate was collectedby vacuum filtration and washed several timeswith absolute ethanol and distilled water, thenvacuum dried at 601C for 6 h. The obtainedsamples were characterized by X-ray powderdiffraction (XRD) using Japan Rigaku Dmaxg-A X-ray diffractometer with graphite monochro-matized Cu-Ka radiation (l ¼ 0:15406 nm). Themicrostructure and morphologies of the samplewere analyzed with transmission electronic micro-scopy performed at 200 kV using Hitachi 800(TEM) and also 200 kV using JEOL 2010(HRTEM).

3. Results and discussion

The XRD pattern of as-obtained sample shownin Fig. 1 can be indexed to face-centered cubicphase PbS and no obvious impurity phase could befound. The shape of the diffraction peaks suggeststhat the sample should be well crystallized. Afterrefinement, the calculated cell constanta ¼ 5:9288 (A is consistent with the literaturedatum (JCPDS 5-592). The broadened nature ofthese diffraction peaks implies that the grain sizesof the sample are nanometer scales. Estimatingfrom the Debye–Scherrer formula, the averagegrain size along [2 0 0] direction is 23.10 nm. Themorphology of the PbS sample indicated from

TEM images (Fig. 2a) is uniform cubic-shapedcrystals with edge length of 22.6 nm. The nano-crystals can arrange themselves into tetragonalnetwork (Fig. 2b). The nanocrystals are well-defined surfaces which can be observed fromhigh-magnification HRTEM image (Fig. 3). Theregular fringes spacing with 0.291 nm correspondto those (0 0 2) lattice planes of face-center cubicphase of PbS. The electron diffraction pattern(inset in Fig. 3) was taken from the selected area ofthe cubic-shaped crystal shown in Fig. 3, using aconvergent lens. The zone axis is [0 0 1] and thetetragonal spots could be indexed to 0 2 0 and %2 0 0of face-centered cubic phase. The facts indicatethat six surfaces of the cubic-shaped nanocrystalsare consisted of {0 0 2} lattice planes. The grainsize along 0 0 2 direction calculated from XRDpattern is quite close to the edge length of tetragonobtained from TEM images, which shows that thenanocrystals should be well-defined cubes.

The hydrothermal reaction of PbS could beexpressed as following:

2PbðAcÞ2 � 2H2Oþ 3S2O2�3 -

2PbSþ 4HAcþ 4SO2�3 þ 2HþþH2O:

In the alkaline condition, disproportionation ofS2O3

2� occurred and produced S2�, which could becombined with Pb2+ to form PbS precipitates. Theionic-surfactant was crucial to the formation ofcubic-shaped PbS nanocrystals. In actual experi-ment, the Pb2+ ions were exchanged into theuniform micelles formed by ionic surfactant inthe solution, then the S2� anions was infused intothe micelles to form PbS nanocrystals. The growth

Fig. 1. XRD pattern of PbS nanocrystals as obtained.

Y. Jiang et al. / Journal of Crystal Growth 231 (2001) 248–251 249

of PbS nanocrystals was confined by the micelles,which resulted in uniform size nanoparticles andmodified cube with (0 0 2) lattice plane as surfaces.This growth pattern was observed by Weller et al.for PbS needles in amphiphilic block copolymermicelles [13]. These uniform cube nanocrystalswere easy to array orderly through Van der Waalsforces as shown in TEM images.

It was found that the size of the PbS nanocrys-tals was effected by the concentrations of Pb2+

and S2O32�, and the detailed preparation proce-

dures. When the Pb(Ac)2 � 2H2O and Na2S2O3

concentration were 0.02M, keeping other condi-tions constant, the sizes of PbS crystals were35 nm. The increase of hydrothermal temperaturecould result in the enlargement of the grain sizes.The surfactant concentration was expected toinfluence the size and shape of nanocrystals, butin our experiment, when molar ratio of Pb(II)/surfactant ranged from 1 to 3, the size and theshape of PbS nanocrystals kept unchanging. Thismight be explained by the formation and stabiliza-tion of surfactant micelle structures in a certainconcentration range.

4. Conclusion

In summary, PbS nanocrystals with uniformcubic-shaped morphology have been successfullyprepared through a hydrothermal process inpresence of ionic surfactant. The nanocrystals arewell-defined cubes with {2 0 0} lattice planes assurfaces. Research for properties of the PbSnanocrystals is in progress. Given the generalityof this approach, we hope to extend our synthetic

Fig. 3. HRTEM image of PbS nanocrystal and corresponding

[0 0 1] axis SEAD pattern (inset).

Fig. 2. TEM images of cubic-shaped PbS nanocrystals (a,b).

Y. Jiang et al. / Journal of Crystal Growth 231 (2001) 248–251250

method to preparation of other uniform, well-defined shape semiconductor nanocrystals.

Acknowledgements

Financial supports from National NaturalScience Fund of China and the 973 Projects ofChina are appreciated.

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