es

entenive

muKSCthan acondSroc

1. Introduction

beenndent on size andmorphology.ium cdiedortants in ligRecentmorphods [3]

2. Experimental section

were dissolved i

Materials Letters 64 (2010) 13571360

Contents lists available at ScienceDirect

Materials

j ourna l homepage: www.e lsthiosemicarbazide both as a sulfur source and as a capping ligand in amethanol/water system [9,10].

Well-dened hierarchical CdS dendrites were synthesized byhydrothermal reaction of CdCl2 and thiourea with appropriatecapping agent at suitable temperatures [11]. In this paper, we reporta new route of controllable synthesis of multi-trunk CdS dendriteswith high yields using CdCl2, KSCN and H2O as the starting materials.To the best of our knowledge, this kind of self-assembled growth ofnovel CdS dendrites by hydrothermal treatment of a Cd2+SCN

complex has not been reported. Based on the experimental results, we

X-ray powder diffraction (XRD) using a Bruker D8 Advance X-raydiffractometer. Transmission electron microscopy (TEM) images andthe corresponding selected area electron diffraction (SAED) patternswere carried out on a Hitachi Model H-800 instrument operating at100 kV. Scanning electron microscopy (SEM) images were obtainedon a JEOL JSM-6700F SEM with an accelerating voltage of 25 kV.

3. Results and discussion

Fig. 1 shows an XRD pattern of the as-obtained product. The strong

found that growth of the novel multi-trunkupon reaction time and nuclei concentratioature. A possible mechanism of crystal grow

Corresponding author. Tel.: +86 531 86180010; faxE-mail address: tangb@sdnu.edu.cn (B. Tang).

0167-577X/$ see front matter 2010 Published by Edoi:10.1016/j.matlet.2010.03.045, wires [4], and tubes [5].-armed CdS crystals. XiedS micropatterns, using

contrast experiments, KSCN was substituted by Na2S9H2O, keepingthe other conditions constant.

The phase purity of the as-synthesized products was measured by

Peng et al. [6] and others [7,8] reported multiet al. have reported a kind of branch-like Ctheir optical properties are highly depeAmong semiconductor materials, cadmtant and have been extensively stuapplications [1,2]. As one kind of impcadmium sulde has broad applicationcells, or other optoelectronic devices.developed to fabricate CdS with novelbeen devoted to the synthesis of CdS rhalcogenides are impor-owing to their desiredsemiconductor material,ht-emitting diodes, solarly, many methods wereologies. Much effort has

two clear solutions weremixed together slowly to yield homogeneousCd2+SCN complex solution, and then it was transferred into a 20-mL Teonlined autoclave. The autoclave was maintained at 160 C for8 h. After the mixture cooled naturally to room temperature, theyellow precipitate was washed with distilled water for several times,and the nal product was dried in a vacuum at 60 C for 4 h. For the-like patterns dependedn at a constant temper-th was proposed.

and narrow peacomparisonwithpeaks can be indwith the standapeak is comparaorientation of th

Fig. 2a showsNumber show

: +86 531 86180017.

lsevier B.V.n two beakers containing 5 mL of distilled water. The

Semiconductor nanocrystals have studied extensively as In a typical experiment, 150 mg CdCl22H2O and 350 mg KSCNGrowth of novel multi-trunk CdS dendritwithout surfactant

Mei Xue, Xiaohua Zhang, Xu Wang, Bo Tang College of Chemistry, Chemical Engineering and Materials Science, Engineering Research CKey Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal U

a b s t r a c ta r t i c l e i n f o

Article history:Received 18 January 2010Accepted 18 March 2010Available online 25 March 2010

Keywords:CdS dendritesSingle crystalHydrothermal system

In the current paper, novelemploying CdCl22H2O andfrom TEM and SEM showedwhich preferentially grew idiffraction (SAED) patternsanalyses proved that the Cresults, a possible growth pby hydrothermal method

r of Pesticide and Medicine Intermediate Clean Production, Ministry of Education,rsity, Jinan 250014, People's Republic of China

lti-trunk CdS dendrites were synthesized via a simple hydrothermal system,N as the starting materials. No extra surfactants were used. The observationst the product composed of a few long central trunks with secondary branches,parallel direction with a denite angle to the trunks. Selected area electronrmed that the dendrite was single crystalline in nature. X-ray diffraction

dendrites were pure hexagonal structure. On the basis of the experimentaless has been discussed.

2010 Published by Elsevier B.V.

Letters

ev ie r.com/ locate /mat le tks show that the material is well crystallized. Bythe data from JCPDS cards No.41-1049, all diffractionexed as a pure hexagonal structure of CdS. Comparedrd reection, the intensity of the (110) diffractiontively strong, which is most probably related to thee CdS crystals.a typical TEM image of a multi-trunk CdS dendrite.s a long central trunk of the individual CdS dendrite.

It is interesting that the secondary branches emerge at about 30 withrespect to the central trunk. The onsets of tertiary branches can beobviously observed in the image of part branches. The tubers ofsecondary branches indicate that the whole architecture resultedfrom the self-growth of the CdS nucleus instead of accumulation ofvarious crystals. From Fig. 2a we also can see the growth regularity of

proved the multi-trunk-shaped structure of the crystals. The imageshows that the product consists almost entirely of such dendriticstructures with mean length of 24 m along the trunks, and thisindicates the high yield and good uniformity achieved with thisapproach. The architectures here have many small secondarybranches on the main trunks. Furthermore, the number of longcentral trunks of an individual dendrite varies from three to ve. TheSEM image in Fig. 2d exhibits the detailed conguration of the noveldendrites. We can see that the two rows of secondary branchesseparate by 180, and the branches in the same row are parallel toeach other emerging at about 30 with respect to the central trunk.

In order to investigate the formation process of the multi-trunkCdS dendrites, time-resolved experiments were carried out and thecorresponding TEM images of the as-prepared samples are illustratedin Fig. S1 (see supporting information). The TEM images in Fig. S2 areall the images of the products prepared by hydrothermal reaction ofCdCl2 and KSCN at 160 C for 8 h, keeping the other parametersconstant. An individual dendrite in Fig. S2a has two trunks. In Fig. S2b,a dendrite has three trunks. In Fig. S2c, dendrites with various shapeswhich have two, three and four trunks are found. Please notice the dotarrow in Fig. S2c, two new trunks are growing. In Fig. S2d, we can seethe ower-like dendrites with shorter trunks and more secondarybranches in many directions. Based on the experimental results, wespeculate that local concentration of the nuclei was necessary for theformation of the novel multi-trunk patterns. When the nucleiconcentration was enough, multi-trunk CdS dendrites formed.

The contrast experiments were carried out to demonstrate thegreat inuence of KSCN on the dendritic morphology. Fig. 3 shows the

Fig. 1. XRD pattern of the products obtained by hydrothermal reaction of CdCl2 andKSCN at 160 C for 8 h.

1358 M. Xue et al. / Materials Letters 64 (2010) 13571360the dendrites. Number shows the second long central trunk havingthe length of 2.2 m, obviously, secondary branches have grown on it.The third central trunk also has appeared, and it already possesses aconsiderable length (see number ). The fourth central trunk hasgrown, too, although it is short and thin (see number ). Moreover,the selected area electron diffraction (SAED) patterns (Fig. 2b)revealed that a long central trunk with secondary and tertiarybranches are single crystalline in nature and the diffraction patterncan be indexed to hexagonal CdS. The SEM image in Fig. 2c resultsFig. 2. TEM and SEM images of the as-prepared products by hydrothermal reaction of CdCl2 ataken from the crystal shown in (a). (c) SEM image of multi-trunk CdS dendrites. (d) SEMSEM images of the products prepared by substituting Na2S9H2O forKSCN, keeping the other parameters constant. From the images, wecan see that the products were hexagonal CdS nanoparticles instead ofelongated CdS crystals. This is because when Na2S9H2O is used as theprecursor, an equilibrium surroundings for crystal growth would besatised, and the crystal morphology was closer to the equilibriumconditions.

In this work, the system contained only three components: CdCl2,KSCN and H2O. No other surfactant or template was needed. Before

nd KSCN at 160 C for 8 h: (a) TEM image of a individual CdS dendrite. (b) SAED pattern

image of a CdS dendrite.

along the [001] direction rather than the [110] direction due to itshigh surface energy. Finally, the branch of the dendrite grows alongthe normal growth direction [001] [11]. After the rst long centraltrunk grew up, the second central trunk, the third central trunk, andeven more central trunks would appear and grow up if theconcentration of nuclei is enough. The schematic view of the growthprocess of the CdS dendrites can be simply described in Fig. 4. Inprinciple, fractal and dendritic growth are diffusion-controlledgrowth, and nonequilibrium growth and molecular anisotropy arethe prerequisites for the formation of dendritic structures [1214].Herein, anisotropy comes from the intrinsical anisotropy of thehexagonal structure CdS [15,16]. Besides, KSCN may act as not onlythe sulfur source but also a ligand to form relatively stable Cd2+NCS

complexes in the initial solution.While the complex ions of SCNwithCd2+ lead to a high remaining monomer concentration after thenucleation stage, thus a nonequilibrium growth for the elongatedcrystals is facilitated [10,17].

1359M. Xue et al. / Materials Letters 64 (2010) 13571360heating, the systemwas a clean and transparent solution. On the basisof the experimental results, a possible growth process of CdS

Fig. 3. (a) SEM images of the as-prepared products obtained by hydrothermal reactionof CdCl2 and Na2S at 160 C for 8 h. (b) High-magnication SEM image of the CdScrystals in (a).dendrites can be simply described as follows. In the initial solution,Cd2+ ions choose sp3 hybridization, SCN can bind Cd2+ to generatethe corresponding complex ionwith tetrahedron conguration.Whenthe system was heated, the interaction between Cd2+ and SCN willbe weakened, and Cd2+ will be released gradually. On the other hand,SCN is attacked by the strong nucleophilic O atoms of H2Omoleculesleading to the weakening of the C=S double bonds, which will bebroken to release S2 anions slowly. Then the active S2 reacts withCd2+ to generate CdS nuclei. Owing to the slow release of reactionions, elongated growth along the [001] direction of rodlike crystals isfavored. Subsequently, some tubers emerge on the side surface, whichare symmetrically separate between each other and initially growalong the [110] direction. However, the CdS crystal prefers to grow

Fig. 4. Schematic view of growth4. Conclusion

In summary, novel multi-trunk CdS dendrites in a pure singlehexagonal phase were successfully synthesized by a simple hydro-thermalmethod, employing CdCl2 and KSCN as the reactants. No othersurfactant was needed during the formation of the dendrite shapedstructures. This interesting morphology of CdS is expected to havenovel properties and may have potential applications in thesemiconductor industry. The simple and convenient method canprobably be expanded to synthesize other inorganic materials withnovel shape.

Acknowledgements

This work is supported by the National Natural Science Funds forDistinguished Young Scholar (No. 20725518), Major Program ofNational Natural Science Foundation of China (No. 90713019). M. Xuethanks Prof. Jiechao Ge and Dr. Yan Geng for SEM measurements.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.matlet.2010.03.045.

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1360 M. Xue et al. / Materials Letters 64 (2010) 13571360

Growth of novel multi-trunk CdS dendrites by hydrothermal method without surfactantIntroductionExperimental sectionResults and discussionConclusionAcknowledgementsSupplementary dataReferences