Available online at SciVerse ScienceDirect

J. Mater. Sci. Technol., 2013, 29(3), 231e236

Preparation and Characterization of Flower-like Cu2SnS3 Nanostructures by

Solvothermal Route

Xiaojuan Liang1), Qian Cai1), Weidong Xiang1,2)*, Zhaopin Chen1), Jiasong Zhong2), Yun Wang1),Mingguo Shao1), Zhenrong Li3)

1) College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, China2) College of Materials Science and Engineering, Tongji University, Shanghai 201804, China3) College of Light Industry, Liaoning University, Shenyang 110036, China

[Manuscript received January 18, 2012, in revised form November 16, 2012, Available online 27 December 2012]

* Corres577 86681005-03JournalLimited.http://dx

Flower-like Cu2SnS3 nanostructures composed of nano-flakes were successfully synthesized by solvothermaltechnique at 180 �C for 16 h. In the preparation process, CuCl2$2H2O, SnCl2$2H2O and thiourea were usedas raw materials, and ethylene glycol were used as solvent. The results showed that the obtained productwas pure phase Cu2SnS3. The average diameter of Cu2SnS3 flowers and the thickness of the nano-flakeswere about 1e1.5 mm and 10 nm, respectively. The influence of reaction time and solvents on themorphology, size and structure of the products was investigated by powder X-ray diffraction and field-emission scan electron microscopy (FESEM). The ultraviolet-visible absorption spectrum measurementindicated that the band gap of the sample was about 1.26 eV and could be applied to the absorbing layer ofthin solar cell. The possible formation mechanism of flower-like Cu2SnS3 was also proposed and discussed.

KEY WORDS: Cu2SnS3 nanostructures; Solvothermal; Semiconductor materials; Formation process

1. Introduction

In recent years, the IeIVeVI ternary group of semiconductorshas attracted much attention because of their interesting prop-erties. Cu2SnS3 belongs to group IeIVeVI series of ternarysulphides semiconductors, which have attracted great attentionowing to their wide applications in photovoltaic devices, light-emitting diode, nonlinear optical materials, photocatalyticactivity and lithium-ion electrode materials[1]. Cu2SnS3 is anemerging semiconducting absorber layer material for solar cellswhich can replace CuInS2 materials owing to its suitable energyband gap (w1 eV), high absorption coefficients, non-toxic,and abundant elements[2]. Cu2SnS3 with interlayer spaces andtunnels in its crystal structure, as well as the well-dispersed anduniform morphology, could be the potentially ideal lithium-ionbattery electrode material[3]. The preparation of non-toxicsemiconductor materials with controllable morphology and sizeis always one of the trends in materials research. There are manymethods to prepare Cu2SnS3 thin films and nanoparticles, such

ponding author. Prof., Ph.D.; Tel.: þ86 577 86596013; Fax: þ869644; E-mail address: xiangweidong001@126.com (W. Xiang).02/$e see front matter Copyright� 2013, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2012.12.011

as conventional solid-state reaction, solvothermal processes,evaporation, spray pyrolysis technique and so on. Onoda et al.[4]

prepared monoclinic Cu2SnS3 by a conventional solid-statereaction which requires elevated temperature, inert atmosphereprotection and a relative long duration. Bouaziz et al.[5] havesynthesized Cu2SnS3 thin film by solid-state reaction undervapor sulfur pressure at 530 �C for 6 h by using a sequentiallydeposited copper and tin layers. Bouaziz et al.[6] also success-fully synthesized Cu2SnS3 thin films by sulfur annealing at550 �C from a superposition of SnS2 and CuxS sprayed thinfilms. Recently, Cu2SnS3 nanocrystals with different morphol-ogies were prepared by solvothermal technique. The sol-vothermal technique is a mild, simple and convenient method toprepare nanomaterials, and it does not need atmosphere protec-tion, high temperature and high pressure. Chen et al.[2]

successfully synthesized Cu2SnS3 nanorods by solvothermalreaction of Cu, Sn, S powders in the temperature range of 250e300 �C for 10e12 h. Qu et al.[3] prepared three dimensionalmesoporous Cu2SnS3 spheres composed of nanoparticles viasolvothermal route by using SnCl4$5H2O, CuCl, thiourea as rawmaterials, PEG-1000 as surfactants, ethanol as solvent. Quet al.[7] also employed SnCl4$5H2O, CuCl2$2H2O as metalprecursor, thioacetamide as sulfur source, PEG-200 as surfac-tants, and successfully synthesized cabbage-like Cu2SnS3nanostructures via solvothermal route, and the individualCu2SnS3 cabbage-like hierarchitecture is constructed from 2D

232 X. Liang et al.: J. Mater. Sci. Technol., 2013, 29(3), 231e236

nanosheets with a thickness of about 15.6 nm. In this work,flower-like Cu2SnS3 nanocrystals were prepared by solvothermalmethod. CuCl2$2H2O, SnCl2$2H2O, thiourea were used as rawmaterials, ethylene glycol as solvent, and no surfactants wereused. Both the cubic and hexagonal Cu2SnS3 were obtained bychanging solvents. The obtained Cu2SnS3 products can be usedas the absorbing layer of thin solar cell after annealing in H2S.

2. Experimental

Fig. 1 XRD spectra of the obtained products prepared at 180 �C fordifferent time: (a) 12 h, (b) 16 h, (c) 20 h, (d) 24 h.

2.1. Synthesis of flower-like Cu2SnS3 nanostructures

All chemicals are of analytical grade and used without furtherpurification. A typical reaction for the preparation of Cu2SnS3flowers can be described as follows: 2 mmol CuCl2$2H2O and1 mmol SnCl2$2H2O were dissolved in 20 ml ethylene glycol,and 3 mmol thiourea was dissolved in 20 ml ethylene glycolseparately, then the sulfur source was added into the metalprecursor solution after being completely dissolved, a homoge-neous milky white solution can be obtained under constantstirring. Then the mixture solution was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 50 ml. Theautoclave was sealed and maintained at 180 �C for 16 h, aftercooling to room temperature naturally, the black precipitate wascollected and washed several times with distilled water andabsolute alcohol to remove the by-products, and then dried undervacuum at 60 �C for 6 h.

2.2. Characterization

The composition and structure of the obtained products werecharacterized by powder X-ray diffraction (XRD) on a Bruker D8-Advance X-ray diffractometer (CuKa radiation, l ¼ 0.15406 nm,scan rate: 0.02�/s, scan range: 10�e80�). The X-ray photoelectronspectroscopy (XPS) was performed on an ESCALAB MKIIX-rayphotoelectron spectroscope (accelerating voltage: 15 kV, AlKaexcitation) and energy dispersive spectrometry (EDS) analysiswas carried out on an OXFORD INCA instrument attached to thescanning electron microscope (SEM, scanning range: 0e20 kV).The field-emission scanning electron microscopy (FESEM) ona JEOL-6700F instrument (accelerating voltage: 10 kV), thetransmission electron microscopy (TEM), high-resolution TEM(HRTEM) and selected area electron diffraction (SAED) carriedout on a JEM-2010 transmission electron microscope (accelera-tion voltage: 200 kV) were used to characterize the morphology ofthe samples. All the measurements were carried out at roomtemperature. The optical properties of the products were deter-mined by means of a UV-2501PC ultraviolet-visible spectropho-tometer (UVeVis).

3. Results and Discussion

In order to understand the crystallographic structure of as-synthesized products, XRD measurements were carried out. It isfound that the Cu2SnS3 phase was not obtained at temperaturebelow 150 �C, but the unidentified amorphous phases formedinstead. Fig. 1 shows the XRD patterns of the products obtained at180 �Cwhen the reaction time is lasted from 12 h to 24 h. It showsthat the crystal phase of Cu2SnS3 is unchanged, and all the peakscorrespond to (111), (220) and (311) crystal planes, which are ingood agreement with the standard data[7] for the pure phase of thecubic Cu2SnS3 structure (JCPDS Card No. 89-2877), and the

(200) crystal plane emerges when the reaction time is extended to24 h. No obvious peaks attributable to other impurities aredetected from this spectrum, confirming that the obtained productsare composed of the pure cubic Cu2SnS3 nanocrystals. Thediffraction peaks turn sharper and the FWHM (full width at halfmaximum) become narrower as the reaction time increases, whichalso indicates the obtained products have higher crystallinity andlarger sizes.Fig. 2 shows the FESEM images of the products obtained at

different reaction time. When the reaction time is 12 h, theobtained products (Fig. 2(a)) are the aggregation of plenty ofnano-flakes with the thickness of 50 nm. When the reaction timeis extended to 16 h, the product is quite uniform in shape andmicrostructure, which consists of large quantities of flower-likeCu2SnS3 nanocrystals with the diameter of about 1e1.5 mm, andthe Cu2SnS3 flowers are constructed by aggregation of large-scale thinner nano-flakes (Fig. 2(b)). As the reaction timeincreases to 20 h, the petals of Cu2SnS3 flowers start to curlfrom the edges of the layers and then a series of microspheresformed. Fig. 2(c) shows that a few flower-like structures anda large amount of Cu2SnS3 microsphere coexist in the samples.When the reaction time is prolonged to 24 h (Fig. 2(d)), all theproducts are composed of microspheres after sufficient durationof the solvothermal treatment, the average diameter of themicrosphere is about 0.5e1 mm, and the lamellar structures onthe surface of microsphere can be obviously observed. It isconcluded that the reaction time plays a critical role in theprocess of synthesizing size adjustable and shape controllableCu2SnS3 nanocrystals.Furthermore, EDS was used to determine the chemical

compositions of the as-prepared flower-like Cu2SnS3, and theresult is shown in Fig. 3. It can be seen from this figure that allpeaks are coincident with Cu, Sn and S elements, respectively.No element of organic matter is detected except the O peaksresulted from the solvent or atmosphere[8]. The average atomicratio of Cu, Sn and S is calculated to be 21.35:12.15:35.35 by thecomparisons of relative areas under the peaks of Cu, Sn and S,which is very close to the stoichiometric ratio of Cu2SnS3.The morphologies and microstructures of Cu2SnS3 flowers

prepared at 180 �C for 16 h were further characterized by TEM,HRTEM and SAED. Fig. 4(a) shows representative TEM imagesof Cu2SnS3 flowers, and it can be observed that the as-productsconsist of flower-like Cu2SnS3 with the diameter of 1 mm whichis in accordance with the FESEM results. Fig. 4(b) represents the

Fig. 2 FESEM images of the obtained products prepared at 180 �C for different time: (a) 12 h, (b) 16 h, (c) 20 h, (d) 24 h.

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HRTEM image of single nano-flake which demonstrates that theCu2SnS3 flakes are constructed by agglomeration of very smallnanoparticles which have the average diameter of 3e5 nm. Thetypical HRTEM image (Fig. 4(c)) of an individual nanoparticleof the nano-flakes reveals that the products are well crystallized,the lattice fringes can be observed clearly, and the regularparallel fringes spacing is about 0.312 nm, which can beassigned to the interplanar spacing of (111) lattice plane of cubicCu2SnS3 crystal. From the SAED pattern shown in Fig. 4(d),these diffraction circles shows obvious single crystalline ringswith identical planar spacing that can be indexed to (111), (200),(220) and (311) reflection direction of the cubic Cu2SnS3, whichare in accord with the four strong diffraction peaks from XRDtest.The valence and composition about the Cu2SnS3 flowers were

further determined by XPS. The spectrum (Fig. 5(a)) indicatesthe presence of Cu, Sn and S, as well as C and O impurities;carbon peak may originate from the organic precursor or refer-ence, and oxygen peak is likely resulted from the solvent or

Fig. 3 EDS spectrum of the samples prepared at 180 �C for 16 h.

atmosphere. No obvious impurities could be detected in thesample, indicating that the level of impurities is lower thanthe resolution limit of XPS (1 at.%). In order to determine thevalence state of each element, high-resolution spectra were alsotaken for the Cu2p, Sn4d and S2p core levels. Fig. 5(b) showsthe XPS spectrum of Cu2p core level, indicating that theobserved values of the binding energies for Cu2p3/2 andCu2p1/2 are about 930.9 and 950.7 eV, respectively, which areconsistent with the values for Cuþ reported previously[9]. Inaddition, the Cu2p3/2 satellite peak characterizing Cu2þ, whichis usually located at 942 eV[10] does not appear in the spectrum,indicating that Cu2þ of the starting material was reduced to Cuþ

during the course of reaction. The Sn3d core (Fig. 5(c)) splitsinto Sn3d5/2 (485.9 eV) and Sn3d3/2 (494.35 eV) peaks, whichall coincide with the reported values for Sn4þ, and no evidenceof Sn2þ which is usually centered at 485.2 eV was detected inthe spectrum[11]. The S2p core level spectrum (Fig. 5(d)) can bedivided into two peaks, which indicates that two chemicalenvironments exist, one is at 161.25 eV corresponding to S fromCueS and the other is at 162.27 eV corresponding to S fromSneS[12]. Hence, the XPS analysis shows the normal valencestates for Cu2SnS3 are Cuþ, Sn4þ, S2�.Fig. 6(a)e(d) shows the XRD patterns of Cu2SnS3 nano-

crystals prepared in the solvent of 1,4-butanediol, dimethylformamide (DMF), glycol and ethanediamine, respectively. TheXRD patterns of all the samples (except the sample prepared byusing ethanediamine as solvent) present similar profiles and allthe diffraction peaks can be well indexed as cubic Cu2SnS3.When ethanediamine is used as solvent, the products do notmatch any of the patterns of the reported CueSneS ternarysulfides, and the diffraction peaks reveal another phase Cu2SnS3that is isostructural with the known hexagonal ZnS pattern,

Fig. 4 TEM (a), HRTEM (b, c) and SAED (d) patterns of the obtained products synthesized at 180 �C for 16 h.

234 X. Liang et al.: J. Mater. Sci. Technol., 2013, 29(3), 231e236

implying the high resemblance for the crystal structure betweenthe as-obtained Cu2SnS3 and wurtzite phase ZnS. The chemicalcompositions of the samples have been investigated by EDS(Fig. 7). The atomic ratio of Cu, Sn and S is calculated to be

Fig. 5 XPS spectra of as-products prepared at 180 �C for 16 h: (a) XPS surveSn3d, (d) core level spectrum for S2p.

21.30:11.13:33.39 by the comparisons of relative areas under thepeaks of Cu, Sn and S, which is very close to the stoichiometricratio of Cu2SnS3, and then basing on the reported results by Wuet al.[13], we can concluded that the obtained product is wurtzite

y spectrum, (b) core level spectrum for Cu2p, (c) core level spectrum for

Fig. 6 XRD spectra of the obtained products prepared in differentsolvents at 180 �C for 16 h: (a) 1,4-butanediol, (b) DMF,(c) glycol, (d) ethanediamine.

Fig. 8 UVevis absorption spectrum of Cu2SnS3 nanocrystals. The insetis a plot of (ahn)2 vs hn.

X. Liang et al.: J. Mater. Sci. Technol., 2013, 29(3), 231e236 235

phase Cu2SnS3 with a hexagonal crystal cell. The possiblereason to form hexagonal Cu2SnS3 is proposed as follows. It isknown that ethanediamine has strong coordination property, andin our present work, the ethanediamine is pivotal for theformation of nanostructured hexagonal Cu2SnS3. It plays threeimportant roles: solvent, reducing agent and ligand. Dissolvingall the reactants in ethanolamine to form a blue homogeneoussolution before heating is crucial for the successful synthesis ofhexagonal Cu2SnS3, in which the blue solution indicates theformation of coordination between Cu2þ and eNH2 of ethane-diamine molecules[14,15]. The result also suggests that differentcoordination properties of solvent affect the phase selection, andthe solvent with strong coordination will lead to the formation ofwurtzite phase Cu2SnS3 nanocrystals.The UVevis absorbance spectroscopy results of Cu2SnS3

nanocrystals are shown in Fig. 8, from which it can be seen thatthe as-prepared Cu2SnS3 nanocrystals present a broad absorptionin a wide wavelength range from UV to visible light. Theabsorption characteristics of the sample obey the model equa-tion: (ahn)2 ¼ B(hn � Eg), where a is the optical absorptioncoefficient, h is Planck constant, n is the photon frequency, B isa constant, and Eg is the energy gap[16]. The band gap energy ofCu2SnS3 nanocrystals was estimated to be 1.26 eV by extrapo-lating straight line of the plot (ahn)2 vs hn in the inset of Fig. 8.This value is close to the optimum for photovoltaic solarconversion in a single-band-gap device.On the basis of the experimental results, the probable

mechanism of the formation for the Cu2SnS3 flowers isproposed. Thiourea molecule can coordinate with metal ions in

Fig. 7 EDS spectrum of the prepared samples under the condition of180 �C for 16 h in ethanediamine.

alkalescency chlorides to form metal-thiourea complexes, and italso can be used to reduce Cu2þ to Cuþ. In the solvothermalprocess, firstly Cu2þ was reduced to Cuþ by thiourea andethylene glycol, and then further complexed by thiourea toform stable Cuþ-thiourea complexes, which held back theformation of copper chalcogenides. At reaction temperature,the complexes decomposed to release Cuþ[17]. BecauseE0Cu2þ=Cuþ (0.17 V) is larger than E0

Sn4þ=Sn2þ(0.15 V), Sn2þ was

slowly oxidized to Sn4þ by Cu2þ under certain conditions.Synchronously, Cu2þ was reduced to Cuþ by Sn2þ. Secondly, itis known that tin chloride dihydrate and thiourea can easilydissolve in ethylene glycol, so during the experimental process,tin ions and sulfur source homogeneously dispersed in themutually soluble ethylene glycol and trace water (fromhydrated tin chloride). Thus, thiourea could react with tracewater to produce H2S slowly, then free Sn4þ reacted withH2S to produce soluble monomeric species [Sn2S6]

4�. Finally,Cuþ combined with [Sn2S6]

4� to form Cu2SnS3 primarynuclei[11,18].The reactions taking place in the system can be described as

follows:

Cu2þ þ 2Tu/½Cuþ ðTuÞ�þþTuþ (1)

Sn2þ þ 2Cu2þ/Sn4þ þ 2Cuþ (2)

CH3CSNH2 þ 2H2O/CH3COOHþ NH3 þ H2S (3)

H2Sþ Sn4þ/ðSn2S6Þ4� þ 12Hþ (4)

Cuþ þ ðSn2S6Þ4�/Cu2SnS3 (5)

Based on the above discussion, a possible formation processof flower-like Cu2SnS3 should involve two stages: an initialnucleating and a subsequent self-growth stage[19-21]. In the firststage, Cu2SnS3 nuclei spontaneously formed by the interactionof raw materials, as depicted in Fig. 9. In the following stage, the

Fig. 9 Depiction of the proposed formation processes of Cu2SnS3 flower.

236 X. Liang et al.: J. Mater. Sci. Technol., 2013, 29(3), 231e236

formation of flower-like Cu2SnS3 was an Ostwald ripeningprocess and the geometric building blocks played a key role inthis process. Because no surfactants were used in our syntheticroute, the primary Cu2SnS3 nuclei in the solution was unstableand there were a large number of dangling bonds, defects or trapson the nuclei surface. With the prolongation of reaction time,the primary Cu2SnS3 nuclei grew larger and preferentiallyaccumulated with each other under the function of stochasticdiffusive force and directive force. Then the nuclei grew aniso-tropically into Cu2SnS3 lamellar nanostructures. Finally thelamellar petals could easily curvature and aggregate to formflower-like Cu2SnS3 microstructure.

4. Conclusion

In summary, flower-like cubic phase Cu2SnS3 crystals with thediameters of 1e1.5 mm have been successfully synthesized bysolvothermal method at 180 �C for 16 h. The Cu2SnS3 flowersconsist of nano-flakes with the thickness of 10 nm. The influenceof reaction time and solvents on the morphology, size andstructure of the products was discussed. The results show thatwurtzite phase Cu2SnS3 nanocrystals can be obtained whenethanediamine is used as solvent, and the reaction time plays animportant role in the synthesis of size adjustable and shapecontrollable Cu2SnS3 nanocrystals. The band gap of obtainedCu2SnS3 is estimated to be 1.26 eV by the measurement of UVevis absorbance spectroscopy, which indicates that the as-prepared Cu2SnS3 flowers can be applied to the absorbinglayer of thin solar cell. Finally, the possible formation mecha-nism of flower-like Cu2SnS3 contains: the formation of metal-thiourea complex and reduction of Cu ion.

AcknowledgmentsThe authors gratefully acknowledge the financial support

from the National Natural Science Foundation of China (GrantNos. 50972107 and 51272059), the Key Scientific and Tech-nological Innovation Teams of Zhejiang Province, China (No.2009R50010), the Natural Science Foundation of LiaoningProvince, China (No. 201202087) and Program of Science andTechnology Project of Wenzhou, China (No. G20110012).

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