Materials Letters 72 (2012) 116–118

Contents lists available at SciVerse ScienceDirect

Materials Letters

j ourna l homepage: www.e lsev ie r .com/ locate /mat le t

A novel method for PbS quantum dot synthesis

Yuechao Jiao, Xiaoyong Gao, Jingxiao Lu ⁎, Yongsheng Chen, Jianpeng Zhou, Xinli LiKey Laboratory of Materials Physics of Ministry of Education, Zhengzhou University, Zhengzhou 450052, China

⁎ Corresponding author. Tel./fax: +86 371 67767803E-mail address: jxlu@zzu.edu.cn (J. Lu).

0167-577X/$ – see front matter © 2011 Published by Edoi:10.1016/j.matlet.2011.12.068

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 October 2011Received in revised form 6 December 2011Accepted 15 December 2011Available online 23 December 2011

Keywords:PbS QDCrystal growthCrystal structureEthylene diamine tetraacetic acid

A novel way to synthesize cubic PbS quantum dots (QDs) with average sizes of about 3 nm to 5 nm is presentedin the current research. A mixture of sodium dodecyl sulfate (SDS) and cetyltrimethyl ammonium bromide(CTAB) was used as stabilizing agent and ethylene diamine tetraacetic acid (EDTA) was used as dispersant inthe chemical bath synthesis method. The role EDTA that plays in the fabrication of PbS QDs was also analyzed.PbS QDs could not be synthesized without EDTA even with the use of stabilizing agents SDS and CTAB. EDTAfacilitates the formation of stable complex compounds with Pb2+ metal ions and prohibits the aggregation ofPbS QDs.

© 2011 Published by Elsevier B.V.

1. Introduction

Owing to size-dependent energy band structures, quantum dots(QDs) of different semiconductor materials have attracted great atten-tion for fabricating functionalmaterials with tunable optical and electri-cal properties [1,2]. Among them, PbSQDs offer a tunable band gap fromthe near-infrared to the visible region by controlling the size of the PbSQDs [3–5]. They have potential applications in photodetector [6], opticalswitches [7], and solar cells [8,9]. Conventionally, PbS was synthesizedby the organ metallic reaction, which required toxic reaction precur-sors, high-boiling point reagents, and high temperature [10]. Recently,somewet-chemical methods have been developed to produce differentPbSnanostructures, such as nanorods [11], nanochips [12], and dendritePbS nanocrystals [13]. However, reports on the synthesis of PbS QDs arelimited. Bakueva et al. [14]was the first to successfully synthesize stableandmonodispersed PbSQDswith diameters of 4 nm±1 nm in aqueoussolutions using a mixture of thioglycerol (TGL) and dithioglycerol(DTG) as the stabilizing agent. Deng et al. [15] synthesized water-soluble PbS QDs in aqueous solution with dihydrolipoic (DHLA) as astabilizer at room temperature. Although PbS QDs can be synthesizedin aqueous solutions using a mixture of TGL and DTG or DHLA as stabi-lizer or surfactant, they are environmentally hazardous, costly, andunsuitable for mass production of PbS QDs. Hence, finding some conve-nient, inexpensive, and suitable stabilizers or surfactants are necessaryfor fabricating PbS QDs.

The combination of PbS QDs and our up-conversion materials wasused to improve the conversion efficiency of solar cells. The presentpaper first presents a novel way to synthesize size-tunable, water-

.

lsevier B.V.

soluble PbS QDs using the mixture of CTAB and SDS as stabilizingagent and EDTA as dispersant under water bath conditions of pH=10and 70 °C. These precursors are nontoxic, odorless, and inexpensivecompared with TGL, DTG, and DHLA.

2. Experimental procedure

PbS QDs were synthesized by chemical bath method. In a typicalprocedure for the preparation of PbS QDs, SDS (0.05 mmol, 99.0%),CTAB (0.05 mmol, 99.0%), and EDTA (0.2 mmol, 99.0%) were dissolvedin the 100 ml deionizedwater previously heated at 70 °Cwithmagneticstirring for about 3 min to 5 min. The pH value of the solution wasadjusted to 10 by adding aqua ammonia dropwise. Next, lead (II) ace-tate solution (10 ml, 0.05 M) was injected to the precursor solutionwithmagnetic stirring for about 1 min to 2 min. Then sulfourea solution(20 ml, 0.025 M) was slowly introduced into the system with continu-ous vigorous stirring for about 10 min. The solution turned from buffto dark-brown, indicating the formation of the PbS QDs. Finally, thereaction solution was cooled naturally at room temperature and theproducts were isolated by subsequent centrifugation, washed threetimes with deionized water and alcohol, and stored in alcohol in thedark or dried in vacuum for about 3 h for TEM or XRD measurement.

The crystalline structure and morphologies of the synthesized PbSQDs were characterized by XRD (Philips PANAlytical X'pert) and TEM(JEM-2100), respectively.

3. Results and discussions

Fig. 1 shows the TEM image of the PbS sample fabricated withoutEDTA. Some irregular particles as well as plate-like PbS nanocrystalsare distributed randomly. When the sulfourea solution is injected to

Fig. 1. TEM image of the PbS sample fabricated without EDTA. The inset denotes theelectron diffraction pattern of the PbS sample.

Fig. 2. (a) TEM image and (b) HRTEM image of the PbS sample fabricated with EDTA.

117Y. Jiao et al. / Materials Letters 72 (2012) 116–118

the reaction system, it first reacts with the water and forms H2S whichdissolves in the water and frees the S2− ions. And then the reactionbetween S2− and Pb2+ takes place rapidly to reach supersaturation,which leads to the explosive nucleation. The chemical reaction involvedcan be described in Eqs. (1) to (3).

NH2CSNH2 þ 2H2O→2NH3 þ H2S þ CO2 ð1Þ

H2Sþ 2OH−→S

2− þ 2H2O ð2Þ

Pb2þ þ S

2−→PbS ð3ÞIt has been documented that in the equimolar SDS/CTAB solution,

relatively larger catanionic vesicles rather than mixed micelles tendedto form [16]. Therefore, a variety of PbSmicrostructures possibly coexistat the early stage of the reaction. Due to the influence of the variousmicrostructures, irregularly shaped nuclei are formed, resulting in thefinal formation of irregular particles as well as plate-like PbS nanocrys-tals. The inset in Fig. 1 indicates that the PbS particles fabricatedwithoutEDTA are well crystallized, and the diffraction spots correspond to thecubic PbS. However, PbS QDs still cannot be synthesized despite themixed SDS and CTAB.

Fig. 2 presents the TEM image and the high-resolution TEM image ofthe PbS sample fabricated with EDTA. The figure indicates that thespherical PbS QDs with good monodispersity are synthesized success-fully, and the particle size mainly lies in the range of 3 nm to 5 nm.The high-resolution TEM (HRTEM) image in Fig. 2(b) shows a clear(200) lattice fringe with a crystal plane spacing of 0.296 nm, revealingthat the PbS QDs are well crystallized.

Compared with Fig. 1, the presence of EDTA plays an important rolein the formation of spherical-shaped PbS QDs. EDTA, an efficient chela-tor for heavy-metal ions, can easily form stable metal ions-EDTAcomplexes. The roles of EDTA in the formation of spherical-shapedPbS QDs can be assumed by applying the LaMer model [17]. On thebase of the model, definite separation of nucleation and crystal growthstages is the primary requirement for small particle formationwith nar-row size distributions, the introduction of chelating agents such asEDTA may have helped to separate the two stages [18,19]. The forma-tion of the metal ions-ETDA complexes reduces the free metal ion con-centration and brings about decrease both in the nucleation process and

in nuclei growth. The chemical reactions for the roles of EDTA are asfollows:

Pb2þ þ EDTA→Pb� EDTA ð4Þ

Pb� EDTAþ S2−→PbS þ EDTA: ð5Þ

During the process of experiments, we observe that the presence ofEDTA postpones the timewhile the color of reaction solution turns frombuff to dark-brown. This phenomenon supports the viewpoint that theformation of the Pb-ETDA complexes can decrease both in the nucle-ation process and in nuclei growth. When sulfourea solution is intro-duced, Pb2+ is discharged from the Pb-EDTA complexes and reactswith S2− to form PbS nuclei. The controlled release of Pb2+ helps toseparate the nucleation and growth stages. But to some extent, thetwo stages lie in different concentration zones. It is reasonable thatthe Pb2+ ions are located in a concentration zone more favorable toparticle nucleation than growth due to the presence of EDTA. The parti-cle is small if the rate of crystal nucleation is faster than that of crystalgrowth. In addition, EDTA also prevents particles from aggregating byshielding the Pb2+, which is crisis for the preparation of goodmonodis-persity PbS QDs.

Fig. 3. XRD pattern of the PbS quantum dots fabricated with EDTA.

118 Y. Jiao et al. / Materials Letters 72 (2012) 116–118

It is well known that the inherent crystal structure of the seeds playsan important role in the formation of nano- and microstructures. Thecubic structure of PbS seeds has isotropic unit cell structures, whichgenerally induce the isotropic growth of particles. The spherical PbSparticles are synthesized by assembly of these smaller PbS nuclei withthe help of the alkyl chain in SDS and micro-micelle of CTAB. Therefore,under the common action of SDS, CTAB, and EDTA, spherical PbS QDsare observed in our experiments.

Fig. 3 presents the XRD pattern for the product fabricated withEDTA. The peaks in Fig. 3 indicate that the product is pure and coincideswell with Joint Committee on Powder Diffraction Standards (No. 05-0592). Hence, PbS QDs are crystallized with a cubic structure.

4. Conclusions

A novel way to synthesize cubic PbS QDs about 3 nm to 5 nm in sizewas developed by chemical bath method using mixed CTAB and SDS asstabilizing agent and EDTA as dispersant. EDTA plays an important rolein the fabrication of PbS QDs owing to separation of nucleation andgrowth stages. In addition, EDTA also prevents particles from aggregat-ing by shielding the Pb2+, which is crisis for the preparation of goodmonodispersity PbS QDs. PbS particles instead of PbS QDs are formedwithout EDTA.

Acknowledgments

This work was supported by the National Basic Research Program ofChina (grant no. 2011CB201600), the National Natural Science Founda-tion of China (grant no. 60807001), and the Foundation of HenanEducational Committee (grant no. 2010A140017).

References

[1] Alivisatos AP. Semiconductor clusters, nanocrystals, and quantum dots. Science1996;271:933–7.

[2] Talapin DV, Murray CB. PbSe nanocrystal solids for n- and p- channel thin filmfield-effect transistors. Science 2005;310:86–9.

[3] Wise FW. Lead salt quantum dots: the limit of strong quantum confinement. AccChem Res 2000;33:773–80.

[4] Cademartiri L, Montanari E, Calestani G, Migliori A, Guagliardi A, Ozin GA. Size-dependent extinction coefficients of PbS quantum dots. J Am Chem Soc 2006;128:10337–46.

[5] Kowshik M, Vogel W, Urban J, Kulkarni SK, Paknikar KM. Microbial synthesis ofsemiconductor PbS nanocrystallites. Adv Mater 2002;14:815–8.

[6] Konstantatos G, Howard I, Fischer A, Hoogland S, Clifford J, Klem E, et al. Ultrasensi-tive solution-cast quantum dot photodetectors. Nature 2006;442:180–3.

[7] Kane RS, Cohen RE, Silbey R. Theoretical study of the electronic structure of PbSnanoclusters. J Phys Chem 1996;100:7928–32.

[8] Zhao N, Osedach TP, Chang LY, Geyer SM, Wanger D, Maddalena T, et al. ColloidalPbS quantum dot solar cells with high fill factor. ACS Nano 2010;4:3743–52.

[9] Pan AC, Cañizo C, Cánovas E, Santos NM, Leitão JP, Luque A. Enhancement ofup-conversion efficiency by combining rare earth-doped phosphors with PbS quan-tum dots. Sol Energy Mater Sol Cells 2010;94:1923–6.

[10] Hines MA, Scholes GD. Colloidal PbS nanocrystals with size tunable near-infraredemission: observation of post synthesis self-narrowing of the particle size distri-bution. Adv Mater 2003;15:1844–9.

[11] Wang SH, Yang SH. Preparation and characterization of oriented PbS crystallinenanorods in polymer films. Langmuir 2000;16:389–97.

[12] Yu DB, Wang DB, Zhang SY, Liu XM, Qian YT. Multi-morphology PbS: frame-filmstructures, twin nanorods, and single crystal films prepared by a polymer-assistedsolvothermal method. J Cryst Growth 2003;249:195–200.

[13] Kuang D, Xu A, Fang Y, Liu H, Frommen C, Fenske D. Surfactant-assisted growth ofnovel PbS dendritic nanostructures via facile hydrothermal process. Adv Mater2003;15:1747–50.

[14] Bakueva L, Gorelikov I, Musikhin S, Zhao XS, Sargent EH. PbS quantum dots with sta-ble efficient luminescence in the near-IR spectral range. Adv Mater 2004;16:926–9.

[15] Deng DW, Zhang WH, Chen XY, Liu F, Zhang J, Gu YQ, et al. Facile synthesis ofhigh-quality, water-soluble, near-infrared-emitting PbS quantum dots. Eur JInorg Chem 2009;2009:3440–6.

[16] Tomašić V, Štefanić I, Filipović-Vinceković N. Adsorption, association and precipita-tion in hexadecyltrimethylammonium bromide/sodium dodecyl sulfate mixtures.Colloid Polym Sci 1999;277:153–63.

[17] Sugimoto T. Preparation of monodispersed colloidal particles. Adv Colloid Inter-face Sci 1987;28:65–108.

[18] Silver J, Martinez-Rubio MI, Ireland TG, Fern GR, Withnall R. The effect of particlemorphology and crystallite size on the upconversion luminescence properties ofErbium and Ytterbium co-doped Yttrium oxide phosphors. J Phys Chem B2001;105:948–53.

[19] Chiu G,Meehan EJ. The preparation ofmonodisperse lead sulfide sols. J Colloid Inter-face Sci 1974;49:160–1.