Materials Research Bulletin - SKKUweb.skku.edu/~nmdl/publication/2012/(MRB_5767_5... · Materials Research Bulletin xxx (2012) xxx–xxx ... A B S T R A C T Pure CuInSe 2 nanoparticles

Materials Research Bulletin xxx (2012) xxx–xxx

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MRB-5767; No. of Pages 5

Synthesis and characterization of CuInSe2 nanoparticles via a solution method

Huiyu Chen a,b,c,*, Ji-Beom Yoo c

a Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, North University of China, Taiyuan 030051, People’s Republic of Chinab College of Materials Science and Engineering, North University of China, Taiyuan 030051, People’s Republic of Chinac School of Advanced Materials Science & Engineering (BK21), Sungkyunkwan University, Suwon 440746, People’s Republic of China

A R T I C L E I N F O

Article history:

Available online xxx

Keywords:

A. Inorganic compounds

A. Nanostructures

B. Chemical synthesis

B. Crystal growth

A B S T R A C T

Pure CuInSe2 nanoparticles have been successfully synthesized via a solution method in the solvent of

oleylamine. Anhydrous InCl3, CuCl, and Se powder were used as the starting materials. The CuInSe2

samples were characterized by XRD, TEM, and XPS techniques. It was found that tetragonal chalcopyrite

structured CuInSe2 nanoparticles were obtained with temperature above 230 8C. Sample prepared at

200 8C possesses triangular morphology and a minute amount of In2Se3 coexists as intermediate. CuInSe2

nanoparticles with size of 20.2 � 0.4 nm were prepared at 230 8C and the narrow size distribution was

ascribed to the employment of hot injection, which was better for homogeneous nucleation. Stable ‘‘ink’’ can

be formed when the as-synthesized CuInSe2 nanoparticles were dispersed in organic solvents such as hexane

and tolune, and such ‘‘ink’’ might have a practical application in CuInSe2-based solar cells.

� 2012 Elsevier Ltd. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

As an important ternary I–III–VI2 semiconductor, copperindium diselenide (CuInSe2) has attracted wide attention inrecent years because it is considered as a promising photovoltaicmaterial for thin film solar cells [1–3]. CuInSe2 has band gap thatmatches well with the solar spectrum, has large absorptioncoefficient and high radiation stability [4,5]. Interestingly, bothp- and n-type of CuInSe2 can be obtained under excess of Se andIn, respectively [6]. However, the fabrication of CuInSe2 thin filmfor the application in solar cells with high efficiency is laborious.It requires precisely control the co-evaporation of severalelements under high vacuum and high temperature conditions.In addition, the post-treatment procedure often involves use oftoxic reagent such as H2Se. Therefore, it is desired to makeCuInSe2 film using CuInSe2 nanoparticles as the startingmaterials and the solution derived CuInSe2 nanocrystal suspen-sions can be easily deposited via spin-coating or printingtechnique. This method might lead to control the stoichiometryover large device areas and thus commercialization of this typeof solar cell is currently underway.

Until now, several methods have been employed to prepareCuInSe2 nanostructures. Typically, CuInSe2 nanowhiskers and

* Corresponding author at: Research Center for Engineering Technology of

Polymeric Composites of Shanxi Province, North University of China, Taiyuan

030051, People’s Republic of China. Tel.: +86 351 3557256; fax: +86 351 3557256.

E-mail address: [email protected] (H. Chen).

Please cite this article in press as: H. Chen, J.-B. Yoo, Mater. Res. Bu

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.04.092

nanoparticles were successfully synthesized via solvothermalroute using anhydrous ethylenediamine as solvent [7,8]. Korgeland coworkers reported that CuInSe2 nanocrystals with trigonalpyramidal shape were prepared with the assistance of sele-nourea as Se source [9]. CuInSe2 nanoparticles and nanoringswere also fabricated in the solvent of oleylamine [10–12], andthe ‘‘ink’’ formed by dispersing these nanostructures in someorganic solvents could be used in CuInSe2-based solar cells. One-dimensional CuInSe2 nanostructures were obtained by electro-deposition approach using porous anodized aluminum oxidetemplate [13,14], or via a Au-catalyzed vapor–liquid–solidgrowth route [15], or by solvothermal method without anyassistance of surfactant [16,17]. Most of the above reportedwork was involved in the complicated synthetic process or useof expensive instruments. Considering that the optoelectronicproperties are closely related to elemental stoichiometry, phasepurity, size, and size distribution of semiconductor nanocrystals,it is expected to synthesize CuInSe2 with high phase purity,excellent crystallinity, and colloidal stability for their practicalapplications.

Herein, we report the synthesis of chalcopyrite CuInSe2

nanoparticles in the presence of oleylamine as solvent. Tempera-ture was found to be a key role for controlling the shape and phaseof these nanoparticles. The ‘‘ink’’ was formed when CuInSe2

nanoparticles were dispersed in nonpolar organic solvents such ashexane and toluene, and such ‘‘ink’’ could be preserved for morethan 3 months without any sedimentation. Therefore, the as-obtained ‘‘ink’’ may serve as an important material for thefabrication of device.

ll. (2012), http://dx.doi.org/10.1016/j.materresbull.2012.04.092


mailto:[email protected]


http://www.sciencedirect.com/science/journal/00255408


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2. Experimental

2.1. Materials

All chemical reagents were purchased from Sigma–Aldrich andused as received without further purification. Copper (I) chloride(CuCl; anhydrous >99.0%), indium (III) chloride (InCl3; anhydrous99.99%), elemental selenium (99.5+%) and oleylamine were used asstarting materials.

2.2. Synthesis of CuInSe2 nanoparticles

In a typical synthesis, 0.25 mmol of InCl3 and 0.25 mmol of CuClwere firstly added to 5 mL of oleylamine, and heated at 80 8C formore than 10 h accompanied by magnetic stirring, in order that thesalts could be completely dissolved. Later on, 0.5 mmol of Sepowder was loaded into 30 mL of oleylamine and heated to 270 8Cwith stirring. The InCl3 and CuCl solution was injected into the hotSe-based solution (hot injection) 30 min later, then let thetemperature of the whole mixed solution cool down to 70 8Cnaturally. After that, the temperature was quickly increased to230 8C within 15 min. After 1 h reaction, the product was collected,rinsed, and finally was dispersed into hexane to form ‘‘ink’’. Thecontrolled experiments were carried out by changing thetemperature and reaction time, respectively, while kept othersynthetic parameters and procedures constant.

2.3. Characterizations

The crystal structure of the obtained product was examined byX-ray powder diffraction (XRD) with a Bruker D8 focus diffrac-tometer at a voltage of 40 kV and a current of 40 mA with a Cu Karadiation (l = 0.15406 nm), employing a scanning rate of 48/m inthe 2u range from 208 to 808. The transmission electron microscopy(TEM and HRTEM) images and the corresponding selected areaelectron diffraction (SAED) patterns were taken on a JEOL JEM-2100F transmission electron microscope at an acceleration voltage

Fig. 1. (a) XRD pattern of the obtained CuInSe2 nanoparticles and in the inset shows the d

and (d) SAED pattern of the CuInSe2 nanoparticles, (e) XPS with survey spectrum and


of 200 kV. X-ray photoelectron spectroscopy (XPS) measurementswere carried out on an ESCA 2000 spectrometer using an Al Ka X-ray as the excitation source.

3. Results and discussion

The crystal structure of the synthesized product was firstcharacterized by X-ray diffraction (XRD). Fig. 1a shows a typicalXRD pattern of the as-obtained CuInSe2 sample synthesized at230 8C for 1 h. All the peaks can be indexed to the tetragonalchalcopyrite structured CuInSe2 (JCPDS No.: 40-1487) and no otherimpurities can be detected. In the inset shows a digital cameraimage of the sample dispersed in hexane, the black ‘‘ink’’ could bepreserved for more than 3 months without any sedimentationindicating the stable status of the CuInSe2 nanoparticles innonpolar organic solvent. There are few reports that CuInSe2

nanoparticles can be preserved over such long duration and thestable ‘‘ink’’ is crucial for the fabrication of solar cell device.

Fig. 1b is a representative TEM image of the resultant CuInSe2

nanoparticles synthesized at 230 8C for 1 h. It can be clearlyobserved that the CuInSe2 nanoparticles exhibit irregular shapesand have uniform size of about 20 nm. The high-resolution TEM(HRTEM) image of CuInSe2 nanoparticle is displayed in Fig. 1c,from which the observed d-spacing is about 0.33 nm and iscomplied with the (1 1 2) lattice spacing of chalcopyrite CuInSe2

nanoparticles. The corresponding selected area electron diffrac-tion (SAED) pattern (Fig. 1d) possesses several rings composedmany discrete spots, and the rings match well with (1 1 2), (2 2 0)/(2 0 4), and (1 1 6)/(3 1 2) reflections of the tetragonal CuInSe2. Inorder to obtain small size of CuInSe2 nanoparticles with narrowsize distribution, it is necessary to employ hot injection and toform homogeneous nucleation. As described in typical synthesis,InCl3 and CuCl salts were initially completely dissolved inoleylamine by magnetic stirring and heating. At 270 8C, thismixture was injected into Se2�-based oleylamine solution bysyringe, and then stopped heating to cool down to 70 8C for betterhomogeneous nucleation.

igital camera image of the ‘‘ink’’ formed in hexane, (b) TEM image, (c) HRTEM image,

(f) Cu 2p core-level spectrum. The sample was synthesized at 230 8C for 1 h.



Fig. 2. CuInSe2 products synthesized at (a) 215 8C, (b) 245 8C, (c) 260 8C, and (d) 275 8C, respectively, with reaction time of 1 h.

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The purity and composition of the obtained CuInSe2 nanopar-ticles were further examined by X-ray photoelectron spectrum(XPS) technique, and Al Ka X-ray was used as the excitation source.Fig. 1e and f shows the XPS survey spectrum and Cu 2p core levelspectrum, respectively. The binding energies obtained in the XPSanalysis were corrected for specimen charging by referencing theC1s to 284.60 eV. The binding energies for Cu 2p3/2 and Cu 2p1/2

(Fig. 1f) are located at 932.6 eV and 952.5 eV, respectively, whichare in agreement with the literature report [8]. In addition, the Cu2p3/2 satellite peak at about 942 eV characterizing Cu2+ does notemerge in the spectrum [7,8]. Therefore, we can conclude that onlyCu+ exists in this compound. The In 3d and Se 3d core level spectra(not shown here) were also consistent with those reported inliterature for CuInSe2 [18].

Temperature was found to be a key role for controlling the size,size distribution, and phase purity of CuInSe2 products. Fig. 2shows the TEM images of the CuInSe2 nanoparticles synthesized atdifferent temperature with reaction time of 1 h. The average sizeincreased with reaction temperature increasing, some particleswith size larger than 50 nm existed in the final products when thetemperature was higher than 260 8C (Fig. 2c and d). Meanwhile, theparticles do not show uniformity as those obtained at lowertemperature (�245 8C). These CuInSe2 nanoparticles exhibit avariety of shapes. It seemed that the size and size distributioncould be less effectively controlled in the higher temperatureregion in this work. It was reported that oleylamine can etchCuInSe2 nanocrystals even in the room temperature [9]. Similaretching of Cu–In–Se nanocrystals was observed in TOP/oleylamineby Allen and Bawendi [19]. Its etching ability could be greatlyenhanced by heating at high temperature. Guo et al. also reported


the formation of hollow CuInSe2 nanorings when TOP/oleylaminwas used as a reaction solvent [10]. In our work, the etching may bestrengthened when the chemical reactions occurred with temper-ature above 260 8C, which destroyed some facets of minuteCuInSe2 nanocrystals in the initial stage. Thus, CuInSe2 nanocrys-tals with various size and shapes could be produced in thesubsequent process. Probably, other factors including the types ofstarting materials, rate of temperature increasing, and so on, mightalso influence it. The detailed reasons need more investigationsand the related work is currently underway.It is interesting to findin Fig. 2a that some triangular CuInSe2 nanoparticles emerged inthe product which was prepared at 215 8C. As a matter of fact, alarge amount of such CuInSe2 nanoparticles with narrow sizedistribution were produced at 200 8C with time of 1 h (Fig. 3a). It isclearly seen from a randomly selected triangular CuInSe2

nanocrystal (Fig. 3b) that the edge length is about 18.6 � 0.7 nm.The fast Fourier transform of the nanoparticle in the inset reveals thesingle crystalline. The measured d-spacings of 0.205 nm correspondto the {2 2 0} lattice planes of tetragonal (chalcopyrite) CuInSe2,which is investigated by the magnified TEM image (Fig. 3c) of theposition circled with while box in Fig. 3b. The XRD pattern (Fig. 3d)demonstrates that minute impurities of In2Se3 coexisted in the finalproduct and it implied that the temperature of 200 8C was not highenough to transform intermediate into pure CuInSe2.

The CuInSe2 nanoparticles synthesized at 200 and 215 8C didnot show obvious difference in size, but the morphology changedto some extent. It indicated that some crystal facets tended topreferentially grow at 200 8C and it resulted in the formation oftriangular shape. CuInSe2 nanoparticles with such novel morphol-ogy were not reported previously. Fig. 4 displays the evolution



Fig. 3. (a) TEM image of triangular CuInSe2 nanocrystals prepared at 200 8C for 1 h, (b) an arbitrary-selected triangular CuInSe2 nanoparticle and in the inset shows the fast

Fourier transform pattern, (c) HRTEM image of the position circled with white box in (b), and (d) XRD pattern of the product.

Fig. 4. CuInSe2 product synthesized at 200 8C for (a) 20 min, (b) 40 min, and (c) 2 h, respectively.

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process of triangular shape by investigating the samples preparedwith different reaction time. When the time was about 20 min, theproduct contained large quantity of small nanoparticles withaverage size smaller than 10 nm (Fig. 4a). We could clearly findthat these nanoparticles have various irregular shapes. Particleswith size of 12.4 � 2.1 nm were produced when the time wasprolonged to 40 min, the triangular shape begin to form at this stageand as viewed by the TEM image in Fig. 4b. A small amount ofirregular CuInSe2 nanoparticles with very large size of 30–40 nmappeared in the final product when we continued to increase thereaction time to 2 h (Fig. 4c). In our synthesis, oleylamine provided asolvating medium for CuCl and InCl3 and served as an effectivecapping ligand. The powder Se was reduced to Se2� in oleylaminesolvent, and the amine group in the molecule was oxidized into a


nitroso group [12]. The detailed reaction pathway needs furtherelucidation.

4. Conclusions

In summary, tetragonal chalcopyrite structured CuInSe2 nano-particles with high purity were prepared via a solution route usinganhydrous InCl3, CuCl, and Se powder as the starting materials.Oleylamine acted as both solvent and reducing reagent. Tempera-ture was found to be a key role for controlling the morphology andphase of CuInSe2 samples. Triangular CuInSe2 nanocrystals withedge length of about 18 nm were produced at 200 8C and nearlymonodisperse nanoparticles could be prepared at 200–245 8C.Broad size distribution exists for CuInSe2 synthesized at or above



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260 8C probably due to the fact that oleylamine can etch theoriginally formed CuInSe2 nanocrystals. The as-obtained CuInSe2

samples are dispersed in hexane to form stable ‘‘ink’’ and can bepreserved for more than 3 months without any sedimentation,which is significant for fabricating the CuInSe2-based solar cells.

Acknowledgments

The financial support from Youthful Science Foundation ofNorth University of China and from the BK21 Project of School ofAdvanced Materials Science & Engineering is gratefully acknowl-edged.

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Materials Research Bulletin - SKKUweb.skku.edu/~nmdl/publication/2012/(MRB_5767_5... · Materials Research Bulletin xxx (2012) xxx–xxx ... A B S T R A C T Pure CuInSe 2 nanoparticles