Synthesis of single crystalline CdS nanowires with polyethylene glycol 400 as inducing template

Available online at www.sciencedirect.comsO1ENCE@inEcT.Transactions ofNonferrous M etals骂PSociety of ChinaScienceTrans. Nonferrous Met. Soc. China 16(2006) 105-109Presswww. csu.edu.cn/ysxb/Synthesis of single crystalline CdS nanowires with polyethylene glycol 400 asinducing templateXU Guo-yue(徐国跃), WANG Han(王函), CHENG Chuan wei(程传伟),ZHANG Hai-qian(张海黔), CAO Jie-ming(曹洁明), JI Guangbin(姬广斌)Nanomaterials Research Institute, College of Materials Science and Technology,Nanjing University of Aeronautics and Astronautics, Nanjing 210016, ChinaReceived 8 May 2005; accepted 9 September 2005Abstraet: Solvothermal technique, an one- step soft solution. processing route was scessfully employed to synthesize single crystallineCdS nanowires in ethylenediamine medium at lower termperature (170 U) for 1-8 d. In this route, polyethylene glycol 400 (PEG400)was used a sufactant, which played a crucial role in preferentially oriented growth of semiconductor nanowires. Characterizations ofas-prepared Cas nanowires by X-ray powder diffraction(XRD), transmission electron microscopy(TEM) indicate that the naonowires,with typical diameters of 20nm and lengths up to several micrometers, have preferential [001] orientation. Also, investigations into thephysical properties of the Cds nanowires were conducted with UV-Vis absorption spectroscopy and photoluminescence emissionspectroscopy. The excitonic photo-optical phenomena of the nanowires shows the potential in the practical applications.Keywords: II -VIsemiconductor; cadmium sulfide; nanowires; single crystal; photoluminescence propertieshave stimulated much research and tecnological1 Introductioninterest[11 - 14]. QIAN's group developed a newchemical technique called solvothermal method toNanostructured materials are of much currentsynthesize CdS nanowires and nanorods[15- 18]. In .interest forextensive potentialapplication inthis communication, we modified the above route andelectronics，optics， catalysis， ceramics, magneticused a different polymer as the inducing template. Asstorage and mechanical devices or in mesoscopiccompared with the templates used in QIAN'sphysics[1l- 3]. In all these nanoscale materials, one-experiments, ours has obvious advantages as follows:dimensional semiconductors such as nanowires(NWs)1) PEG400 has thermnal stability and unhydrolyzationand nanorods(NRs)， have demonstrated significantproperties, which makes it more environmentallypotential as fundamental building blocks forfriendly. In addition, PEG400 is more common, whichassembling a range of nanoelectronic and nano-could reduce the cost of experiments and simplify thephotonic device[4- 6] including room-temperatureexperiments process; 2) PEG 400 can be easilyfiel-effect transistor(FETs)[7, 8], p-n diodes[8, 9],separated from the products only by deioned water andbipolar junction transistor, and recently, complemen-absolute alcohol washing, so it is able to obtain hightary inverter[9] which represents a key component forpurity single crstalline CdS nanowires; 3) Thelogic. HU et al[10] also presented substantial promiseobtained CdS nanowires have much smaller diameters,for fabricating integrated nanosystems. CdS, as awhich is very important to the mesoscopic propertiestypical wide band-gap II - VI semiconductor, has aand fabrication of nanodevices[19]. Using cadmiumband-gap of 2.42 eV at room temperature. It has manynitrate and thiourea as starting material, we havecommercial or potential applications in light-emittingsuc中国煤化lell-dispersed singlediodes, solar cell, or other optoelectronic devices. ThecrysF/pical diameters of 20synthesis and study of CdS nanowires and nanorodsnmYHC N M H Grometers.Foundatfon item: Projcct(J1300A002) supported by the National Defense Fundamental Fund of China; Project(51410040305hk0214) supported by theWeaponry Equipmeot Pre-research Foundation of China; Prject(90505008) supported by the National Natural Science Foundation of ChinaCoresponding author: XU Guo-yue; Tel: +86-25-84894320; Fax: +86- 25-84892951; E-mail: xuguoy@ Dua.edu.cn106XU Guo yue, et al/Trans. Nonferrous Met. Soc. China 16(2006) 105-109XRD patterns of the products obtained at2 Experimentaldifferent reaction times are shown in Fig.1. In Fig.l,the curves from (a) to (d) represent the products2.1 Chemical reagent and reactorsprepared in varied reaction times: 1, 2, 4 and 8 d,All the chemicals used in this route were reagentrespectively, All the peaks could be indexed as agrade without further purification. Cd(NO3)●4H2Otypical hexagonal phase (wurtzite structure) Cds. Theand thiourea were provided as Cd and S source,calculated cons- tants a=4.136 A and c=6.713 A are .respectively. Polyethylene glycol 400 (PEG400) waswell consistent with the reported data for CdS (JSPDSused as the inducing soft template during the synthesisCard File No.06-0314). All the curves in Fig.1 showprocess. The bidentate ligand ethylenediamine oftenthat the (002) peak is comparatively sharper than thatused in solvothermal synthesis under supercriticalof the other peaks and has preferential [001]conditions was chosen as the reaction medium.orientation, which indicates these Cas nano-Commercial Teflon-lined stainless-steel autoclaves ofcrystallities grow along this direction.50 mL capacity were used as the reactor.3.2 Effects of reaction time on synthesis of CdS2.2 Synthesis procedurenanowires0.001 mol Cd(NO3)2 ●4H2O was dissolved intoFig.1 shows the XRD patterns of the products15.0 g polyethylene glycol 400 gel (PEG400, 5.0 g;obtained in varied reaction times from 1 d to 8 d. Itdeionized water, 10.0 g), kept at room temperature forcan be seen in Fig.1 that as the reaction time is24 h in order to achieve good dispersion of Cd+ in theprolonged from 1 d to 4 d, the (100) and (101) peakspolymer matrix. 0.001mol thiourea was dissolved inalmost keep unchanged, while the (002) peak becomes25 mL ethylenediamine, and then the PEG gel dosedsharper and sharper. This result indicates that when thewith Cdi+ was mixed with the ethylenediamine solvent.reaction time is prolonged, the Cds nanocrystallitiesThe mixture was put into the Teflon-linedmostly grow along the [001] orientation rather thanstainless- steel autoclave of 50 mL capacity. The tankother directions. That is, when the reaction time iswas maintained at 170 C for 1- 8 d and allowed toprolonged, the CdS nanowires would grow longer andcool down to room temperature. The yellow productslonger along the [001] orientation with diameterobtained were washed with absolute alcohol andremaining nearly unchanged. It also can be seen fromdeionized water for several times. The sample wasthe patterns when the reaction time is furtherdried in vacuum at60 C for4- 5 h. .prolonged above 4 d, the nanowires grow litle or stopgrowing. This result is consistent with those by TEM2.3 Characterizationand SAED observations, which we will discuss later.The products obtained were characterized byX-ray powder diffaction(XRD) patterns employing ascanning rate of 0.02(° )/s in the 2 0 range from含10° to 80°，using a Bruker D8 Advance difracto-meter equipped with graphite monochromatized CuKaradiation ( λ=1.5405 A) operated at 40 mA and 40 kV.()Transmission electron microscopy (TEM) andselected-area electron diffraction(SAED) observationswere conducted on a JEM-200CX transmission人人(C)|electron microscope, using an accelerating voltage of200 kV. The sample was dissolved in acetone andAndispersed to suspension by an ultrasonic disperser.a)」Room-temperature UV-Vis measurement was madewith a HP-6100 UV-Vis scanning spectrophotometer0 20304(506070 80in 300- - 600 nm wavelength range using 1cm quartz20/<)cuvettes. The photoluminescence emission(PL) spec-trum was taken at room temperature with a Hitachi850 Spectrofluorophotometer.Fi中国煤化工es prepared at diferetreac;(6)2 d;(c)4d; (d) 8dYHCNMHG3 Results and discussion3.3 TEM and SAED observationsTEM images for the samples obtained for3.1 XRD measurementsXU Guo-yue, et alTrans. Nonferrous Met. Soc. China 16(2006) 105-109107different reaction times(1- -4d)at 170 C are shownin Fig.2. Fig.2 shows that all the CdS nanorystallitiesare homogeneous nanowires or nanorods with uniform(a)diameters of about 20 nm and lengths up to severalmicrometers. In addition, it can be seen from Fig.2that the length of Cds nanowires grows longer andlonger, which indicates the aspect ratio of theas-prepared CdS nanowires is becoming higher andhigher. The TEM observation is consistent with theresult by XRD analysis. The as-prepared CaSnanowires may aggregate together sometimes, but the100 nmwell-dispersed single crystalline nanowires can beeasily found by TEM everywhere. Fig.3(a) shows atypical single crystalline CaS nanowire for the sampleobtained in 4 d, and it has a smooth and straightmorphology. The selected area electron difractionpattemn(SAED) (Fig.3(b)) reveals the single crystallinenature of the as-prepared CdS nanowires, which canbe indexed to the strong reflection of wurtzite Cds[001].Fig.3 TEM images of typical Cas nanowire(a) and sclected-area electron diffraction patterm of wurtzite Cds nanowire(b)3.4 UV-Vis absorption spectroscopy and photo-luminescence(PL) spectrumThe optical properties of the CdS nanowires areboth attractable and remarkable. The UV-visabsorption and photoluminescence(PL) spectrum 0Cas nanowires are very complex and their explanationb)has attracted some controversies.The UV_Vis absorption spectrum for the CdSnanowires obtained for 4 d is shown in Fig.4. Thesample was dispersed in absolute ethanol by ultrasonicdisperser. The curve shows an obvious peak at 500 nm.It was reported that only the presence of a narrowcluster size distribution may cause the presence of thisspectrum feature[20, 21]. The absorption edge for thebulk hexagonal CdS is at 512 nm(2.42 eV). Comparedwith the bulk CdS, it is believed that the blue shift inabsorption peak was obviously caused by the quantumconfinement effect.Fig.5 shows the photoluminescence(PL) emissionspectrum of an absolute ethanol solution containingCdS nanowires that is obtained by present route for 4d. The patterm consists of one strong and narrowemission. at 340 nm using a 310 nm excitationwave中国煤化工at 340 nm may beattrittransition in CdSCNMHGcrysi:MHis kind of band-edgeFig.2 TEM images of CdS nanowires prepared by present routeluminescence is caused by the recombination offor different reaction times at 170 U:(a) I d; (b)2d; (c)4dexcitons and/or shallowly trapped electron-hole108XU Guo-yue, et al/Trans. Nonferrous Met. Soc. China 16(2006) 105-109pairs[21]. The apparent blue shift and the strong peakNotablely, the polyethylene glycol 400 plays anare also indicatives of the size quantization ofimportant role in one-dimensional growth of CdSas-prepared CdS nanowires.crystallites, in the absence of which only short nanorodscan be obtained[23]. Cd2+ ions were well dispersed inhe polymer matrix.The polymer may connected withthe ethylenediamine to form chain structures with manypores. These pores are connected and continuous. So in500the process of the CdS formation, PEG-400 gel servedas a molecular template in control of the CdS crystalgrowth. At the early stage,only small nanocrystals form,with prolonged duration, these small nanocrystalsevolve into long nanowires along the polymer chains.The time effects in the nanowires growth were provedby XRD and TEM analyses. Comparatively, the simpleelectrochemical deposition into anodic alumina only350450550600gets short CdS nanorods or nanoparticles with wideWavelength/nmdistribution of lengths and widths, moreover the CdSFig.4 UV-Vis absorption spectrum of CdS nanowires obtainednanorodsarepolycrystalline[24].Thesameby present route for 4 dsolvothermal method, which uses polyacrylamide(PAA)as inducing template, results in much thicker widths(about 40 nm) of the Cds nanowires, causing a relative340low aspect ratio[17, 18].4 ConclusionsSingle crystalline Cds nanowires with straight andsmooth morphology have been successully synthesizedby treatment of Cd2+ well dispersed in PEG400 gel withthiourea in ethylenediamine medium at 170 C forseveral days. The Cds nanowires obtained are singlecrystalline nature and continuous. Significantly, the240280320360400length of the CdS nanowires can be controlled byregulating the reaction time. These continuous singleFig.5 Photoluminescence emission (hex =310 nm) spectrum ofcrystalline Cds nanowires may have many applicationsCdS nanowiresin optoelectronic devices, which are under process inour future work.A possible mechanism for the formation of the CdSnanowires was presented[l5 - - 18]. In our syntheticAcknowledgementsystem, the investigations into CdS nanowire formationWe thank Professor HONG Jian-ming of Nanjingindicated that the nucleation and growth were wellUniversity for use of the TEM and PL measurements.controlled. Firstly, ethylenediamine(en), as a stronglybidentating solvent, was ready to form relatively stableReferencesCd2+ complexes(ogB2 =10.09). Next, the thiourea wasinclined to decompose at such a high temperature(170[1] ALIVISATOS A P. Seraiconductor clusters,nunocrystals. aoC) and generate s2 ions slowly and homogeneously.quantum dots[J]. Science, 1996, 271: 933-937.The S- ions will react with the Ca4 ions that has[2] WANG Y, HERRON N. Nanometer- sized semiconductor clusers:materials synthesis, quantum size effects, and photophysicalchelated with ethylenediamine in a reversible andpropries[J]. J Pbys Chem, 1991, 95<2): 525- 532.effective pathway. The overall reaction process could be[3] CALVERT P. Nanotube composites- a recipe for strength[]. Nature,ilustrated by Eqns.(1)[2] and (2):中国煤化工un ler dose.(NH2)2CS+2OH~台CH2N2 +H2O+s2-，2e2-nanocohesion[J]. Phys Rev Lett, 1997, 79: 2863- -2866.Cd2+口Cd(en)2口CdS + 2en(2)HOPPLEER C. ZWERGER W. Quntum fuctutions in the cohesiveforce of mallic nanowires[J]. Phys Rev B, 199,9 59: R7849 -R7851.XU Guo-yue, et al/Trans. Nonferrous Met. Soc. China 16(2006) 105-109109I7] CUI Y, DUAN x, Hu 1, LIEBER C M. Doping and ecrical transport[17] ZHAN Jin-hua, YANG Xiao guang. WANG Dun-wei.in silicon nanowires[J]. 1 Phys Chem B, 2000, 10422): 5213- 5216.Polymer-controled growth of CdS nanowires[{]. Adv Mater, 2000,[8] DUAN x HUANG Y, CUI Y, WANG J F, LIEBER C M. Indium12(18); 1348- 1350.phosphide nanowires as building blocks for nanoscale electrouic andoploelectronic devices[J]. Nature, 2001, 409(6816); 66-69.chemical solution transport mecbanism for one dimensional growth of9] CUI Y, LIEBER C M. Functional nanoscale electronic devicesCdS nanowires[J]. J Grystal Growth, 2000, 220: 231- 234.assembled using silicon nanowire builing blocks[D]. Science, 2001,[19] PAN z W, XIE S s, CHANG B H, WANG CY, LUY, LIU W,zHoU291:851-853.W Y,LI W z, QIAN L x. Very long carbon nanoubes[J]. Nature, 1998,[10] HU J, ODOM T W, LIEBER C M. Chemistry and physics in one394(6694): 631-632.dimension: syothesis and properties of nanowires and nanotubes[J]- Acc[20] MODES s, LIANOS P. Wavelength dependence of the photochemistyChem Res, 1999, 32: 435-445.of hydrogen iodide- acetylene complexes in solid krypton[D} J Phys[1]LIY,XUD,ZHANGQ,CHEND,HUANGF,XUY,GUO0,GUZChem, 1989. 93: 5834.Preparation of cadnium sulfide nanowire arrays in anodic alurninum[21] SAPANHEL L ANDERSON M A. Syathesis of porous quantum)-sizeoxide templates[J]. Chem Mater, 199 11(12): 3433- :3435.cadniurn sulfide menbranes: photoluminescence phasc shift and[12] DUAN X, LIEBER c M.General syothesis of cormpounod semiconductordemodulation measurements[J]. J Am Chem Soc, 1990, 112(6): 2278-nanowires[]. Adv Mater, 2000 12(4): 298- 302.2284.[13] ZHANG Peng, GAO Lian. Synthesis and characterization of CdS[22] YUS H, YOSHMURA M, MARIAI, CALDERON M, FUJIWARA T,nanorods via hydrothemal microemulsion[J]. Langmuir, 2003, 19(1):FUJINO T, TERANISHI R. In situ fabrication and optical properties of208-210.a novel polystyrene/scmiconductor nanocomposite embedded with Cas{141 SHAO Ming-wang, XU Fen, PENG Yi-ya, WU J， LI Qing.nanowires by a soft solution processing route[J] . Lingmuir, 2001,17(5): 1700- 1707.at room temperature[]. New J Chem, 2002, 26(10): 1440- 1442.[23] LI Ya-dong, LIAO Hong-wei, DING Yi, QIAN Yi-tai, LI Yang. CUI[15] WU Ii, JIANG Yang, u Qing, LIU Xiao ming, QIAN Y-tai. UsingEn-zhou. Nonaqueous synthesis of Cds nanorod semiconductof[].thiosemicarbaride as starting material to synthesize Cds crystallineChem Mater, 1998, 10(9): 2301 -2303.nanowhiskers via solvotbermal route[I. J Crystal Growth, 2002, 235:[24] ROUTKEVITCH D， BIGIONI T， MOSKOVTTS M, et al.421- 424.Electrochemical fabrication of CdS nanowine arrays in porous anodic[16] YANG Jian, ZENG Jinghui, YU Shu-hong, YANG L, ZHOU 0 E,aluruinum oxide templatcs[]. 1 Phys Chem,1996, 1033): 14037-QIAN Y T. Formation process of Cas nanorods via solvothermal14047.route[]. Chem Mater, 200 12(1): 3259- -3263.(Edited by LI Xiang-qun)中国煤化工MYHCNMHG