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Synthesis of hexagonal structured wurtzite and chalcopyrite CuInS2 via a simple solution route
Nanoscale Research Lettersvolume 6, Article number: 562 (2011)
Wurtzite semiconductor CuInS2 [CIS] has been reported in recent years. As a kind of metastable structure, it is a great challenge to synthesize pure wurtzite CIS at low temperature. In this paper, via a simple and quick solution route, we synthesize both wurtzite- and chalcopyrite-structure CIS. Well-controlled wurtzite CIS hexagonal plates are obtained when an appropriate agent is added. The influence of the used agent triethanolamine [TEA] has also been studied, and it turns out that without TEA, chalcopyrite CIS with a kind of rare morphology is formed through this method.
Ternary • I-III-VI2 semiconductors have garnered great interest due to their promising photovoltaic applications [1, 2]. Meanwhile, the growing need for highly efficient and low-cost photovoltaic devices continues to drive new research in developing non-vacuum techniques. Thus, solution routes to fabricate I-III-VI2 semiconductor nanocrystals have been greatly developed because nanocrystal synthesis can utilize lower-cost processing and device fabrication can benefit from roll-to-roll or solution-phase processing such as spin-coating [3–10]. Among the kinds of I-III-VI2 semiconductors, one good example is CuInS2 [CIS] which has high optical absorption coefficient (>105 cm-1) and desirable bandgap (approximately 1.45 eV) that matches well with solar spectra [11, 12]. Therefore, researches on nanocrystal CIS synthesis have attached great attention [13–18].
It was reported that CIS has three crystal structures [19, 20]: these are (1) the chalcopyrite structure [CH-CIS], stable from room temperature to 1,253 K; (2) the zinc-blende structure, stable between 1,253 and 1,318 K; and (3) an unknown structure, existing from 1,318 K to melting temperature. It was found that the unknown structure could be turned into wurtzite phase, while Cu and In atoms occupy the cation sublattice positions disorderly . Moreover, previous studies have proved that both the zinc-blende and wurtzite structures are metastable at room temperature since they may transform into chalcopyrite phase as the temperature recurred to room temperature [19, 20]. As a result, CH-CIS is believed to be the most common phase and extensively used in CIS solar cells.
Recently, Pan et al. reported the synthesis of zinc-blende- and wurtzite-structure CIS [WZ-CIS] nanocrystals by a hot-injection method , which brings great interest in these two structures, especially in WZ-CIS. Since wurtzite phase allows flexibility of stoichiometry, it provides the ability to tune the Fermi energy over a wide range, which is beneficial for device fabrication . Some groups have reported to synthesize WZ-CIS nanocrystals in recent years [23–29]. Most reported works are also synthesized in an oil system since former researches find out that solvents like ethanolamine, ethylenediamine, and isopropanolamine are beneficial for the formation of WZ-CIS . However, it is still a challenge for the synthesis of metastable WZ-CIS at room temperature, especially via a simple method and low-cost precursors.
In our study, we synthesize pure and well-controlled WZ-CIS via a simple and quick solution route in a polyalcohol system under low temperature. We find that the agent triethanolamine [TEA] plays an important role in the synthesis of WZ-CIS phase. Without TEA, large-diameter hexagonal-structure CH-CIS is obtained. It is a kind of rare morphology in CH-CIS since most other groups pay more attention to controlling the sizes and shapes of the particles and focus on the synthesis of nanosheets, nanorods, quantum dots, and others [13–18]. The growth process and mechanism of hexagonal-structure CH-CIS are discussed.
All the reagents are used as received without any further purification. The reagents are as follows: copper(I) dichloride [CuCl 2H2O], indium(III) trichloride [InCl3 4H2O], thiourea [TA], diethylene glycol [DEG], and TEA.
Nanostructured WZ-CIS samples are synthesized via a simple and quick solution route. One mmol of CuCl 2H2O and 1 mmol of InCl3 4H2O are dissolved into 40 mL DEG in a three-neck flask. This solution is stirred under N2, while the temperature rises to 180°C. After adding 3 mL TEA while stirring, the solution turns into a deeper color but still clear without any precipitation. Then, stoichiometric amounts of TA dissolved in 10 mL DEG is slowly added into the former solution. After a 2-h reaction at 180°C, the flask is removed from the heater and cooled at room temperature. The precipitates are separated by centrifugation, washed with ethanol for three to five times, and dried at 80°C for 5 h. Meanwhile, CH-CIS samples are synthesized via a similar route, but without adding TEA.
The as-prepared products are characterized by X-ray diffraction [XRD], scanning electron microscopy [SEM], and transmission electron microscopy [TEM]. XRD is carried out to study the crystal structures of all the samples by using an X'Pert PRO (PANalytical, Almelo, The Netherlands) diffractometer equipped with a Cu Kα radiation source. Data are collected by step-scanning of 2θ from 10° to 70° with a step of 0.02° and a counting time of 1 s per step. Morphology of the products is investigated by SEM and TEM. The SEM images are taken by SEM S4800 (Hitachi, Tokyo, Japan). The TEM images and high resolution TEM [HRTEM] are acquired by Tecnai F20 (FEI, Hillsboro, OR, USA).
Results and discussion
Structure characterization of WZ-CIS
The products are studied by XRD to refine the structure. A calculated pattern using the lattice parameters reported by Pan et al. (unit cell dimensions a = b = 3.897 Å, c = 6.441 Å and space group: P63mc)  has a good match to the experimental XRD pattern (Figure 1), which indicates that the products are wurtzite-structure CIS. All the diffraction peaks have a good match to the previous reported wurtzite CIS pattern , and no chalcopyrite CIS phase is determined.
The morphologies of the samples are examined by TEM and HRTEM. Figure 2 shows the typical images of the products, which indicate that the products are hexagonal nanostructured plates. The average diameter of the plates is 80 to 100 nm, and the thickness is about 30 nm. This kind of nanostructures has also been reported by other groups using solvothermal methods or other oil system solvents [21, 24]. The HRTEM image (Figure 2b) exhibits clear lattice fringes with a spacing of 0.345 nm, which is well matched to the interplanar spacing of (002) plane of the WZ-CIS.
The influence of TEA
A former study has reported that solvents like ethanolamine, ethylenediamine, and isopropanolamine are beneficial for the formation of WZ-CIS [24, 25]. The pivot is that these solvents play important roles as ligand and reducing agent which reduce Cu2+ to Cu+. Qi et al. have synthesized WZ-CIS in 2009 using ethanolamine as solvent . They claim that the successful synthesis of WZ-CIS strongly depends on the formation of coordination between Cu2+ or Cu+ and -NH2. In our work, TEA is the pivotal agent. TEA is a kind of versatile ligand that readily forms coordination compounds with almost all metal ions . It can play a similar role as ethanolamine, which is coordinating with Cu+ and facilitating the formation of WZ-CIS. Also, the experiment phenomena show clearly that only when the Cu source and TEA have an appropriate ratio and form a clear solution before the S source is added can pure WZ-CIS be obtained. Otherwise, when the precursor solution is turbid, CH-CIS will co-exit with WZ-CIS. Moreover, TEA also provides ligand for In3+ to limit the size and control the morphology of the products. It can be presumed that TEA coordinates with Cu+ and In3+, changing and controlling the relationship of release rates between these two cations, and makes Cu+ and In3+ occupy the cation sublattice positions disorderly when they react with S2-.
Figure 3 shows the XRD and SEM results of the products using the same synthesis method, but not adding TEA. In comparison with those with TEA (as shown in Figures 1 and 2), two significant differences can be found. First, without using TEA, the products appear as chalcopyrite phase. As seen from the XRD pattern (Figure 3b), all the diffraction peaks are perfectly matched with tetragonal lattice CIS (JCPDS PCPDFWIN, No. 85-1575). Second, the hexagonal plate morphology has remained, but the plates are much bigger in size and have a diameter of 2 to 3 μm and a thickness of about 100 nm (Figure 3a). The HRTEM image (Figure 3d) exhibits (112) orientation of CH-CIS with a d-spacing of 0.32 nm. These two differences indicate that the -NH2 in TEA molecules plays a key role to synthesize WZ-CIS and control the size of the CIS hexagonal plates.
The growth process of CH-CIS hexagonal plates
As CH-CIS belongs to a tetragonal lattice, to investigate the growth process of the hexagonal plates, the samples obtained after different reaction times are characterized by SEM and TEM (Figure 4a, b, c, d, e, f). It can be seen that the hexagonal structure is fabricated by self-aggregated nanoparticles about 100 nm in size. Both the SEM and TEM images (the insert of Figure 4) verify this self-aggregation process. As the reaction is going on, the aggregated nanoparticles recrystallize from the center of the plates, forming a smooth surface (the dark central part of Figure 4c). After 30-min reaction, most of the products are hexagonal plates (Figure 4e).
The XRD patterns of the products at different reaction times shown in Figure 5 explain the growth process further. After 5-min reaction, most of the diffractions that peak in the XRD pattern are indexed to hexagonal phase CuS with CH-CIS phase. As the time is expanding, the diffraction peaks of CH-CIS get stronger while the peaks of CuS become weaker. After 20 min, nearly pure phase CH-CIS is formed with little CuS existence. This kind of reaction process has also been reported by other groups in the synthesis of flower-like CIS particles . It indicates that hexagonal CuS is formed first. CuS nanoparticles grow into small flakes, which exhibit the growth characteristic habit for hexagonal CuS. Then, CuS flakes self-aggregate to form hexagonal plates while at the same time, they react with In3+ to form CIS. As the time is expanding, CuS decreases while CIS increases. At the end, the CH-CIS pure phase is formed.
In conclusion, WZ-CIS with a well-controlled hexagonal structure is synthesized via a simple and quick solution route. It is found that the agent TEA plays a key role in forming the wurtzite phase and controlling the size of the products since it coordinates with Cu+ and also provides ligand for In3+ to limit and control the morphology of the product. Without TEA, the products appeared as CH-CIS phase, and the hexagonal structures had much larger diameters. The growth process shows that the fabrication of CH-CIS hexagonal structures is due to hexagonal phase CuS self-aggregation and reaction with In3+. After a short time, about 20 min, nearly pure CH-CIS phase with hexagonal structures is formed.
Habas SE, Platt HAS, van Hest MFAM, Ginley DS: Low-cost inorganic solar cells: from ink to printed device. Chem Rev 2010, 110: 6571. 10.1021/cr100191d
Editors Call for papers: special issue on chalcopyrite thin film solar cells Prog Photovolt: Res Appl 2008, 16: 271.
Panthani MG, Akhavan V, Goodfellow B, Schmidtke JP, Dunn L, Dodabalapur A, Barbara PF, Korgel BA: Synthesis of CuInS 2 , CuInSe 2 , and Cu(In x Ga 1-x )Se 2 (CIGS) nanocrystal "inks" for printable photovoltaics. J Am Chem Soc 2008, 130: 16770. 10.1021/ja805845q
Norsworthy G, Leidholm CR, Halani A, Kapur VK, Roe R, Basol BM, Matson R: CIS film growth by metallic ink coating and selenization. Solar Energy Materials & Solar Cells 2000, 60: 127. 10.1016/S0927-0248(99)00075-6
Hibberd CJ, Chassaing E, Liu W, Mitzi DB, Lincot D, Tiwari N: Non-vacuum methods for formation of Cu(In, Ga)(Se, S) 2 thin film photovoltaic absorbers. Prog Photovolt: Res Appl 2010, 18: 434. 10.1002/pip.914
Long F, Wang W, Tao H, Jia T, Li X, Zou Z, Fu Z: Solvothermal synthesis, nanocrystal print and photoelectrochemical properties of CuInS 2 thin film. Materials Letters 2010, 64: 195. 10.1016/j.matlet.2009.10.044
Weil BD, Connor ST, Cui Y: CuInS 2 solar cells by air-stable ink rolling. J Am Chem Soc 2010, 132: 6642. 10.1021/ja1020475
Guo Q, Kim SJ, Kar M, Birkmire WNSRW, Stach EA, Agrawal R, Hillhouse HW: Development of CuInSe 2 nanocrystal and nanoring inks for low-cost solar cells. Nano Letters 2008, 8: 2982. 10.1021/nl802042g
Chen G, Wang L, Sheng X, Yang D: Cu-In intermetallic compound inks for CuInS 2 solar cells. J Mater Sci: Mater Electron
Redinger D, Molesa S, Shong Y, Farschi R, Subramanian V: An ink-jet-deposited passive component process for RFID. IEEE Trans Electron Devices 2004, 51: 1978. 10.1109/TED.2004.838451
Bandyopadhyaya S, Chaudhuri S, Pal AK: Synthesis of CuInS 2 flms by sulphurization of Cu/In stacked elemental layers. Solar Energy Materials & Solar Cells 2000, 60: 323. 10.1016/S0927-0248(99)00064-1
Klaer J, Bruns J, Henninger R, Siemer K, Klenk R, Ellmer K, Bräunig D: Efficient CuInS 2 thin-film solar cells prepared by a sequential process. Semicond Sci Technol 1998, 13: 1456. 10.1088/0268-1242/13/12/022
Zhong H, Zhou Y, Ye M, He Y, Ye J, He C, Yang C, Li Y: Controlled synthesis and optical properties of colloidal ternary chalcogenide CuInS 2 nanocrystals. Chem Mater 2008, 20: 6434. 10.1021/cm8006827
Chang C, Ting J: Phase, morphology, and dimension control of CIS powders prepared using a solvothermal process. Thin Solid Films 2009, 517: 4174. 10.1016/j.tsf.2009.02.037
Kruszynska M, Borchert H, Parisi J, Kolny-Olesiak : Synthesis and shape control of CuInS 2 nanoparticles. J Am Chem Soc 2010, 132: 15976. 10.1021/ja103828f
Castro SL, Bailey SG, Raffaelle RP, Banger KK, Hepp AF: Synthesis and characterization of colloidal CuInS 2 nanoparticles from a molecular single-source precursor. J Phys Chem B 2004, 108: 12429. 10.1021/jp049107p
Courtel FM, Hammami A, Imbeault R, Hersant G, Paynter RW, Marsan B, Morin M: Synthesis of n-type CuInS 2 particles using N-methylimidazole, characterization and growth mechanism. Chem Mater 2010, 22: 3752. 10.1021/cm100750z
Han S, Kong M, Guo Y, Wang M: Synthesis of copper indium sulfide nanoparticles by solvothermal method. Materials Letters 2009, 63: 1192. 10.1016/j.matlet.2009.02.032
Binsma JJM, Giling LJ, Bloem J: Phase relations in the system Cu 2 S-In 2 S 3 . J Cryst Growth 1980, 50: 429. 10.1016/0022-0248(80)90090-1
Abrahams SC, Berbstein JL: Piezoeletric nonlinear optic CuGaS 2 and CuInS 2 crystal structure: sublattice distortion in AIBIIICVI 2 and AIIBIVCV 2 type chalcopyrites. The Journal of Chemical Physics 1973, 59: 5415. 10.1063/1.1679891
Qi Y, Liu Q, Tang K, Liang Z, Ren Z, Liu X: Synthesis and characterization of nanostructured wurtzite CuInS 2 : a new cation disordered polymorph of CuInS 2 . J Phy Chem C 2009, 113: 3939. 10.1021/jp807987t
Pan D, An L, Sun Z, Hou W, Yang Y, Yang Z, Lu Y: Synthesis of Cu-In-S ternary nanocrystals with tunable structure and composition. J Am Chem Soc 2008, 130: 5620. 10.1021/ja711027j
Connor ST, Hsu C, Weil BD, Aloni BD, Cui Y: Phase transformation of biphasic Cu 2 S-CuInS 2 to monophasic CuInS 2 nanorods. J Am Chem Soc 2009, 131: 4962. 10.1021/ja809901u
Koo B, Patel RN, Korgel BA: Wurtzite-chalcopyrite polytypism in CuInS 2 nanodisks. Chem Mater 2009, 21: 1962. 10.1021/cm900363w
Norako ME, Franzman MA, Brutchey RL: Growth kinetics of monodisperse Cu-In-S nanocrystals using a dialkyl disulfide sulfur source. Chem Mater 2009, 21: 4299. 10.1021/cm9015673
Lu X, Zhuang Z, Peng Q, Li Y: Controlled synthesis of wurtzite CuInS 2 nanocrystals and their side-by-side nanorod assemblies. Cryst Eng Comm
Bera P, Seok S: Facile synthesis of nanocrystalline wurtzite Cu-In-S by amine-assisted decomposition of precursors. Journal of Solid State Chemistry 2010, 183: 3669.
Kruszynska M, Borchert H, Parisi J, Kolny-Olesiak J: Synthesis and shape control of CuInS 2 nanoparticles. J Am Chem Soc 2010, 132: 15976. 10.1021/ja103828f
Batabyal SK, Tian L, Venkatram N, Ji W, Vittal JJ: Phase-selective synthesis of CuInS 2 nanocrystals. J Phys Chem C 2009, 113: 15037. 10.1021/jp905234y
Karadag A, Yilmaz VT, Thoene C: Di- and triethanolamine complexes of Co(II), Ni(II), Cu(II) and Zn(II) with thiocyanate: synthesis, spectral and thermal studies. Crystal structure of dimeric Cu(II) complex with deprotonated diethanolamine, [Cu 2 (μ-dea) 2 (NCS) 2 ]. Polyhedron 2001, 20: 635. 10.1016/S0277-5387(01)00720-3
Zheng L, Xu Y, Song Y, Wu C, Zhang M, Xie Y: Monodisperse CuInS 2 hierarchical microarchitectures for photocatalytic H 2 evolution under visible light. Inorg Chem 2009, 48: 4003. 10.1021/ic802399f
This work is supported by the National Basic Research Program of China or the 973 Program (No. 2007CB613403), the Innovation Team Project of Zhejiang Province (2009R50005), and the Fundamental Research Funds for the Central Universities.
The authors declare that they have no competing interests.
All the authors contributed to the writing of the manuscript. XS and YL carried out the experiments under the instruction of LW and DY. Also, the financial support was provided by DY. All the authors read and approved the final manuscript.