[2]Synthesis and characterization of copper sulfide nanocrystal

Synthesis and characterization of copper sul?de nanocrystal with three-dimensional ?ower-shape

Jing Zou ?Jianxue Zhang ?Baohua Zhang ?Pingtang Zhao ?Xiaofeng Xu ?Jun Chen ?Kaixun Huang

Received:12December 2006/Accepted:6June 2007/Published online:27July 2007óSpringer Science+Business Media,LLC 2007

Abstract New copper sul?de nanocrystals with three-dimensional (3D)?ower-shape were synthesized by using copper acetate (Cu(ac)2)and citric acid (cit)and thiourea (Tu)as precursors at 160°C in an anhydrous ethanol by a solvothermal route.The structure and properties of as-prepared products were characterized by X-ray powder diffraction,transmission electron microscopy,?eld emis-sion scanning electron microscope and scanning electron microscope.The optical properties of copper sul?de nanocrystals were examined by UV–vis and FTIR.The crystal growth mechanism was also proposed.

Introduction

Design of new nanobuilding blocks and architectural con-trol synthesis of nanocrystals with well-de?ned morphol-ogy in superstructure materials have attracted signi?cant interest of chemist and material scientist,because of their unique optical,electrical and catalytic properties that rely sensitively on both size and shape of the samples are completely distinct from the bulk materials [1,2].

For the past few years,various efforts have been made to control the architecture and spatial patterning of metal chalcogenide semiconductor particles.Copper sul?de is a

good prospective optoelectronic material.It has potential applications in solar cells,electrochemistry cells,IR detectors,sensors,and catalysts [3–13].For these appli-cations,a variety of techniques [14–21]have been devel-oped to prepare copper sul?des with controllable microstructural morphologies such as nanoparticles,hol-low spheres,?akes,rods,wires,belt or ?owerlike struc-tures [22–27].However,copper sul?de has very complex crystal chemistry owing to its ability to form various stoichiometric compounds.Mixture phase copper sul?des are usually obtained in many synthetic routes.

In this paper,a new solution approach to synthesize well-de?ned morphology of CuS nanocrystals with 3D ?owerlike at low temperatures by a simple solvothermal process in anhydrous ethanol was reported.These CuS nanocrystals with 3D ?ower-shape were assembled by thin uniform single-crystalline nanosheets with thickness of ~3–6nm and showed very strong UV bandage and IR absorption.

Experiment

For all the reactions in this study,the mixture of copper acetate monohydrate (Cu(ac)2)áH 2O and citric acid (cit)was used as Cu sources,thiourea (Tu)(H 2NCSNH 2)was used as sulfur sources,respectively.In a typical process,the reactant molar ratio (C R )of Tu:Cu(ac)2,2mmol Cu(ac)2and 4mmol cit was dissolved in 40mL anhydrous ethanol;in another container,6mmol Tu was dissolved in 40mL anhydrous ethanol.The Cu solution was added to the thiourea solution dropwise at room temperature with continuous stirring for 30min.Then the mixture was transferred into a 100mL Te?on-lied autoclave for solvothermal processing at 160°C for 8h in an electric

J.Zou áP.Zhao áK.Huang (&)

Department of Chemistry,Huazhong University of Science and Technology,Wuhan,Hubei 430074,P.R.China e-mail:hxxzrf@https://www.sodocs.net/doc/c32061155.html,

J.Zou áJ.Zhang áB.Zhang áX.Xu áJ.Chen

School of Chemical Engineering and Pharmacy,Wuhan Institute of Technology,Wuhan,Hubei 430074,P.R.China

J Mater Sci (2007)42:9181–9186DOI 10.1007/s10853-007-1923-0

desiccation box.After this process,black crystalline products were collected by centrifugation and thorough washings with water and ethanol for several times then dried at50°C in air for4h.The X-ray powder diffraction (XRD)pattern of the samples was characterized using a Japan XD-5A X-ray diffractometer with Cu K a radiation (k=1.5406A?′)and a scanning rate of0.08degree/s in2h ranges from15°to70°;EDX spectrum was recorded at EDAX-FALCON60;scanning electron microscope(SEM) and?eld emission scanning electron microscope(FESEM) was carried out on a JEOL JSM-5510LV and a FEI Sirion 200,respectively.The powder samples were dispersed into ethanol and then placed on the copper wafers for SEM and FESEM observation.Transmission electron microscopy (TEM)images and selected-area electron diffraction (SAED)patterns were obtained by a transmission electron microscope(JEOL JEM-2010)with an accelerating voltage of200kV.A drop of the solution was dried on a holey copper grid for TEM and SAED observation.The optical property tests of the samples were carried out on a HP-8452A UV–vis absorption spectrometer and FTIR absorp-tion spectrometer from Nicolet Impact420.

Results and discussion

Figure1shows a typical XRD pattern of as-prepared CuS nanocrystals.All the re?ections could be indexed to the pure hexagonal CuS in a good agreement with the reported data(JCPDS06-0464;a=3.791,c=16.34).No impuri-ties could be detected in this pattern,which implies that pure CuS could be obtained under the current synthetic route.The strong relative intensity of the[107]peak indicated preferentially orientation effects of the growth direction of nanosheet of hexagonal copper sul?de[19].The EDX result(Fig.2)demonstrates two elements of only Cu and S contained in the sample;the Cu:S atomic ratio was determined to be1:1,suggesting that pure copper sul?de was fabricated.

Figure3a shows a typical SEM image of CuS nano-crystals obtained by the solvothermal route via2mmol Cu(ac)2and4mmol cit solution reacting with6mmol Tu solution at160°C for8h.Perfect CuS nanocrystals with 3D?ower-shape were assembled by many ultrathin and uniform nanosheets.Careful examination reveals that these CuS nanosheets are3–6nm in thickness and CuS nano-?owers are2–5l m in diameter.The products were further characterized by TEM(Fig.3b).The corresponding SAED pattern(inset Fig.3b)exhibits polycrystalline rings of copper sul?de.To give a further understanding of these ?owers assembled,the samples were intensively sonicated for20min in ethanol.Many individual petal and residual CuS nano?owers could be found in the sonicated sample. The TEM image of a residual CuS nano?ower and typical copper sul?de sheet after sonication is shown in Fig.3c. The corresponding SAED pattern(inset Fig.3c)implies that the individual nanoslice is single crystals with pure hexagonal copper sul?de.This veri?es further that perfect CuS nanocrystals with3D?ower-shape are assembled by many spokewise single-crystalline nanostructured sheets.

It was reported that many factors had signi?cant effects on the shapes and sizes of the halcogenide nanocrystals in previous studies[28,29].In this study,?ve major factors including the reactant molar ratio(C R)of Tu:Cu(ac)2,the reactant concentration,temperature,medium pH value and reaction time play important roles to control the mor-phology and sizes of copper sul?de nanocrystals.Figure4 shows typical SEM images of CuS crystals for different C R.While the C R=0.5,1,2,3,3.2and4at160°C for8h, respectively,the size and morphology of copper sul?de nanocrystals were quite different.The shapes of

copper Fig.2EDX spectrum of as-prepared copper sul?de nano?owers

sul?de crystals were changed from particle to ?ower shape gradually.Flower morphology of copper sul?de nano-crystals was formed at the C R ?3.However,perfect CuS nanocrystals with 3D ?ower-shape was obtained at C R =3.Figure 5shows typical SEM images of different initial concentration of reactant with C R =3at 160°C for 8h.The different shapes of CuS crystals were formed at

different initial reactant concentrations.Only Fig.5c shows perfect 3D ?ower shape at 0.05mol/L Cu(ac)2.The representative SEM images of the samples obtained from different medium pH value with C R =3at 160°C for 8h are shown in Fig.6.The perfect CuS nanocrystal with 3D ?ower-shape was largely dependent on medium pH values.CuS particles or dollops were found at pH =2,the conglobation of CuS particles with diameter of 22um–10um were produced at pH =3,rulelessparticles were found at pH =8.Only medium pH with 4was in favor of ideal 3D ?ower shape formation of CuS nanocrystals.However,the XRD patterns recorded for these samples indicate that they are hexagonal phase.

In addition,the morphologies of CuS nanocrystals were sensitive to solvothermal temperature.The reactions at C R =3were carried out at different temperatures,i.e.,150,158,160,162and 170°C.Flower morphology of CuS nanocrystals was obtained at 158–162°C for 8h (data not shown),but perfect ?ower morphology was produced only at 160°C.Thus ideal CuS nanocrystals with three-dimensional ?ower-shape were obtained at temperatures controlled strictly.

To understand the possible formation mechanism of the copper sul?de nanocrystals,the growth process of the nanocrystals was monitored by SEM and FTIR.The SEM images of the samples obtained after the reaction for 1,

1.5,

Fig.3SEM,TEM and SAED images of as-prepared copper sul?de products with 3D ?ower-shape.(a )SEM;(b )TEM;(c )TEM and

SAED

Fig.4SEM images of as-prepared copper sul?de

products of different proportion C R .(a )0.5;(b )1;(c )2;(d )3;(e )3.2;(f )

4

Fig.5SEM images of as-prepared copper sul?de products of different concentrations Cu(ac)2with C R =3at 160°C for 8h:(a )0.25mol/L;(b )0.10mol/L;(c )0.05mol/L;(d )0.025mol/L

1.75,2and 8h at 160°C are shown in Fig.7and their FTIR spectra are shown in Fig.8.These images clearly exhibit the evolution of CuS ?ower shape over the periods of reaction time.At ?rst,the ruleless sticks crystal of Cu–Tu complex with total length ranging from 260nm to 850nm was formed at 1h (Fig.7a),then those ruleless sticks crystals were congregated (Fig.7b).The character-istic IR spectrum of the product obtained for 1h (Fig.8b)is comparable with one of Tu.Because the c N–H absorption bands in the region 3,100–3,400cm –1and d N–H 1,618cm –1in the spectrum of Tu were not shifted to lower frequencies on the formation Cu–Tu complex,it is concluded that Cu to N bonds are not present and that the bonding must be between Cu and S atom.It can be seen from spectrum Fig.8a,b that the c C=S at 1,413cm –1and 730cm –1of Tu are shifted to low frequencies 1,396cm –1and 705cm –1,respectively in Cu–Tu complex.Similarly the C–N stretching vibration at 1,084cm –1is shifted to higher fre-quency 1,108cm –1.This also shows that binding of Cu with Tu is through S [30,31].When the reaction proceed for 1.75h,the similarly ?ower with diameter of about

0.8l m was formed (Fig.7c).The basic ?ower shape with diameter of about 1l m was produced for 2h (Fig.7d).Continuously increasing the reaction time for ~8h,perfect ?ower shape with diameter of ~2.5um was fabricated (Fig.7e).Meanwhile,the IR spectrum of the products prepared for 1.75,2and 8h (Fig.8c)shows a strong absorption in main IR ?eld and presence of any impure composition was not observed.This result is supported by spectra of XRD and EDX.

Morphology and sizes control of CuS nanocrystals are accomplished by carefully controlling nanocrystal growth parameters such as reactant molar ratio (C R ),the reactant concentration,temperature,medium pH value and reaction time.The results suggest that the particle sizes and their shape depend on the nucleation rate,the growth rate and the growth habit of crystals.The nucleation rate and the growth rate of CuS nanocrystals with 3D ?ower-shape were well controlled by growth parameters in the present system.

On the basis of the above results,it could be believed that the formation of ideal ?ower morphology of

CuS

Fig.6SEM images of as-prepared copper sul?de

products of different acidity.(a )pH =2;(b )pH =3;(c )pH =4;(d )pH =6;(e )pH =

8

Fig.7SEM images of as-prepared copper sul?de products of different times.(a )1h;(b )1.5h;(c )1.75h;(d )2h;(e )8h

nanocrystals had followed a thermolysis of Cu–Tu pre-cursor congeries and a ripening process of the crystals.One the one hand,as copper ions prefer square coordination with cit,the resulting extended chains can be connected into 2D layers through the coordination of hydroxyl ions to d z 2orbitals of copper atoms.Thiourea molecules got at-tached with the cation Cu(cit)22+as start materials with plane structure which easily leads to the formation of resembled morphology of nanoparticle [32].On the other hand,the hexagonal crystal nucleus of CuS are produced by a thermolysis of Cu–Tu precursor congeries.Further increasing reaction time,the thermodynamic regime gov-erns the growth process and Cu–Tu complex continuously supply monomers on the [107]faces with low surface en-ergy,and therefore promote the growth in the [107]direction of a ?at structure.Hence,CuS nanosheets along the [107]plane [19]can readily intermesh each other to form a 3D ?ower-shape of CuS nanocrystals along with process of Ostwald.

The reaction mechanisms involved are:Cu 2tt2Cit !?Cu eCit T2 2t

e1T

NH 2CSNH 2t2H 2O !2NH 3tCO 2tH 2S e2T

?Cu eCit T2 2ttH 2S !CuS t2H tt2Cit

e3T

n CuS !?CuS n e4T

The optical properties of as-prepared CuS nanocrystals at room temperature were studied and were shown in

Fig.9a.The UV–vis absorption spectrum of 3D-?ower-shaped of CuS nanocrystals exhibits a well-de?ned absorption feature at 224nm.This is considerably blue-shifted relative to the bulk value of 245nm (Fig.9b).

Conclusions

CuS nanocrystals with 3D ?ower-shape and hexagonal phase were synthesized via simple and free-template solvothermal method at low temperature.The CuS nano-crystals with 3D ?ower-shape were formed with a mass of nanosheets of interlacing arrangement.Moreover,the size and morphology of as-prepared products could be con-trolled by domination of reaction condition,including the molar ratio and concentration of reactant,reaction tem-perature,medium pH value,solvothermal reaction time.

Acknowledgments We thank the faculties from the Analysis and Test Center of Huazhong University of Science and Technology and Center for Electron of Microscopy of Wuhan University for the technical assistance on characterization.

References

1.Chen X,Nazzal A,Goorskey D,Xiao M (2001)J Phys Rev B 64:245304

2.Xin ZL,Jian BX (2002)J Phys Rev B 66:115316

3.Li M,Schnablegger H,Mann S (1999)Nature 402:393

4.Rodriguez JA,Jirsak T,Dvorak J,Sambasivan S,Fischer D (2000)J Phys Chem B 104:319

5.Huang MH,Mao S,Feick H,Yan H,Wu Y,Kind H,Weber E,Russo R,Yang P (2001)Science 292:1897

6.Mann S,Ozin GA (1996)Nature

382:313

Fig.8IR spectra of as-prepared copper sul?de products.(a )thiourea;(b )1h;(c )8

h

Fig.9UV–vis spectrum of copper sul?de nanocrystals (a )and bulk copper sul?de (b )

7.Yang H,Coombs N,Ozin GA(1997)Nature386:692

8.Ahmadi TS,Wang ZL,Green TC,Henglein A,ElSayed MA

(1996)Science272:1924

9.Lindroos S,Arnold A,Leskela M(2000)J Appl Surf Sci158:75

10.Erokhina S,Erokhin V,Nicolini C(2003)J Langmuir19:766

11.Reijnen L,Meester B,Goossens A,Schoonman J(2003)J Chem

Vap Deposition9:15

12.Sy¨etkus A,Galdikas A,Mironas A,Sy¨imkiene I,Ancutiene I,

Janickis V,Kaciulis S,Mattogno G,Ingo MG(2001)J Thin Solid Films391:275

13.Blachnik R,Muller A(2000)J Thermochim Acta361:31

14.Parkin P(1996)J Chem Soc Rev25:199

15.Yi HC,Moore JJ(1990)J Mater Sci25:1159

16.Lu J,Zhao Y,Chen N,Xie Y(2003)J Chem Lett32:30

https://www.sodocs.net/doc/c32061155.html,rsen TH,Sigman M,Ghezelbash A,Doty RC,Korgel BA

(2003)J Am Chem Soc125:5638

18.Dong X,Potter D,Erkey C(2002)J Ind Eng Chem Res41:4489

19.Zhang P,Gao L(2003)J Mater Chem13:2007

20.Xu CQ,Zhang ZC,Ye Q,Liu X(2003)J Chem Lett32:198

21.Liao XH,Chen NY,Xu S,Yang SB,Zhu JJ(2003)J Cryst

Growth252:59322.Lu Q,Gao F,Zhao D(2002)J Nano Lett2:725

23.Chen X,Wang Z,Wang X,Zhang R,Liu X,Lin W,Qian YJ

(2004)J Cryst Growth263:570

24.Wang C,Tang K,Yang Q,Bin H,Shen G,Qian Y(2001)J Chem

Lett30:494

25.Gorai S,Ganguli D,Chaudhuri S(2005)J Cryst Growth Des

3:876

26.Ni Y,Liu H,Wang F,Yin G,Hong J,Ma X,Xu Z(2003)J Appl

Phys A10:1007

27.Qin AM,Fang YP,Ou HD,Liu HQ,Su CY(2005)J Cryst

Growth Des3:856

28.Peng ZA,Peng X(2001)J Am Chem Soc123:1389

29.Joo J,Na HB,Yu T,Yu JH,Kim YW,Wu F,Zhang JZ,Hyeon T

(2003)J Am Chem Soc125:11100

30.Angelimary PA,Dhanuskodi S(2001)J Cryst Res Technol

36:1231

31.Yang J,Zeng JH,Yu SH,Yang L,Zhang YH,Qian YT(2000)J

Chem Mater12:2924

32.Wells AF(1983)Structural inorganic chemistry,5th edn.

Clarendon Press,Oxford,p1113