Quaternary chalcopyrite (Cu2CoSnS4, Cu2ZnSnS4) nanocrystals displaying high crystallization and controlled morphology were synthesized via a high-temperature growth regime achieved by dissolution–reprecipitation of tailored ultrafine precursors in the temperature range 400–500 °C

Balocchi, Andréa

Chane-Ching, Jean-Yves

Gillorin, Arnaud

Marie, Xavier

Zaberca, Oana

English

International audienceQuaternary chalcopyrite (Cu2CoSnS4, Cu2ZnSnS4) nanocrystals displaying high crystallization and controlled morphology were synthesized via a high-temperature growth regime achieved by dissolution–reprecipitation of tailored ultrafine precursors in the temperature range 400–500 °C

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Highly-crystallized quaternary chalcopyrite nanocrystals via a high-temperature dissolution–reprecipitation route

J. Y. Chane-Ching

A. Gillorin

O. Zaberca

A. Balocchi

X. Marie

Crossref

Chemical Communications

Open Archive Toulouse Archive Ouverte

                         To link to this article: DOI:10.1039/C1CC10749F         URL: http://dx.doi.org/10.1039/C1CC10749F           This is an author-deposited version published in: http://oatao.univ-toulouse.fr/Eprints ID: 5612 To cite this version:   Chane-Ching, Jean-Yves and Gillorin, A. and Zaberca, O. and Balocchi, A. and Marie, X. Highly-crystallized quaternary chalcopyrite nanocrystals via a high-temperature dissolution–reprecipitation route. (2011) Chemical Communications, vol. 47 . pp. 5229-5231. ISSN 1359-7345 Open Archive Toulouse Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.   Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@listes.diff.inp-toulouse.fr  Highly-crystallized quaternary chalcopyrite nanocrystalsvia a high-temperature dissolution–reprecipitation routeJ. Y. Chane-Ching,*a A. Gillorin,a O. Zaberca,a A. Balocchib and X. MariebDOI: 10.1039/c1cc10749fQuaternary chalcopyrite (Cu2CoSnS4, Cu2ZnSnS4) nanocrystalsdisplaying high crystallization and controlled morphology weresynthesized via a high-temperature growth regime achieved bydissolution–reprecipitation of tailored ultrafine precursors in thetemperature range 400–500 1C.Nanostructured cells are generally described as third-generationsolar cells with the goal to significantly increase deviceefficiencies.1 These cells include quantum-dot intermediateband solar cells,1 hybrid cells2 and involve the use of semi-conducting inorganic nanocrystals. Recently, use of thesesemiconducting nanocrystals has been extended as inks3to replace more expensive non-vacuum techniques to thefabrication of thin film absorbers of second-generation cells.In these applications, surface and bulk properties of thesenanocrystals are crucial for improving the efficiency of thenano-crystal-based devices. Regarding the bulk properties,electron–hole recombination which increases current losses,is very sensitive to the presence of defects. Thus, single-crystallineor highly crystallized nano-crystals are highly desirable forthese photovoltaic applications. Indeed, a crucial point toincrease efficiency of nanostructured solar cells is the abilityto control the defect concentration of the semiconductornanocrystals.4 However, the limited available informationrelating to the defect concentration dependence of the photo-voltaic properties is mainly due to the difficulty in preparingperfectly crystallized inorganic semiconducting nanocrystals.Therefore, it is important to develop new synthetic routesproducing size and morphology controlled nanocrystals whichexhibit high crystallization. Among candidate materials,ternary chalcopyrite nanocrystals5 have attracted great attentionbecause of their potential for use in solar cells. Most recently,quaternary chalcopyrite nanocrystals6 displaying similarstructure and optical properties to CuInS2 are receivingconsiderable attention as promising candidates for low-costabsorber layers.7 Cu2ZnSnS4 is one of the well-knownquaternary chalcopyrites,8 however, few reports describe thesynthesis and properties of these nanocrystals.6 The onlyreported synthetic routes,6,9 involve the arrested precipitationin coordinating solvent at temperatures of 200 1C. Never-theless, UV-Vis absorption spectra recorded on the resultingnanocrystals did not exhibit any well-defined excitonic peak.The lack of a distinct excitonic peak could arise from variousfactors such as a large variation in size and composition of thequaternary chalcogenide nanocrystals, or from a too largedefect concentration existing in these semiconductor nano-crystals, prepared at a relatively low temperature. Herein, weemploy a high temperature (450 1C) dissolution–reprecipitationroute performed in molten salts to sufficiently increase thecrystallization of the nano-crystals such that a distinctexcitonic peak can be observed in the UV-Vis absorptionspectrum. Critically, the multiplication of nucleation centersresulting from the use of tailored ultrafine Cu2MSnS4(M = Co, Zn) precursors affords to preserve the nanometricsize of the crystals during the high-temperature crystallizationstage. Moreover, a high-temperature growth regime wasachieved for the size and morphology control of theCu2MSnS4 nanocrystals via the use of a sulfide-source moltensalt such as potassium thiocyanate, KSCN.In our high-temperature crystallization process, high-temperature growth kinetics is achieved via the selection of amolten salt allowing a fine tuning of supersaturation conditions.Indeed, thermal decomposition of the melt KSCN occurs attemperatures >275 1C,10 with cyanogen, sulfide and sulfurproduction. Thus, formation of quaternary chalcopyriteinvolves oxidation of the sulfur by the cyanide anion, withformation of the sulfide anion. A preliminary set of controlexperiments aimed at the formation of Cu2MSnS4 (M=Co, Zn)particles was performed using metal or metal oxides asmetal precursors in KSCN and thiourea, CS(NH2)2. Asexpected,11,12 all these experiments yielded only either micro-crystalline or fully aggregated crystals. In general, nucleationand growth in dissolution–reprecipitation processes aresignificantly dependent on precursor dissolution kinetics,permitting size and morphology control of Cu2MSnS4 nano-crystals. Thus, in a first stage, ultrafine precursors displayingCu2MSnS4 single phase and small primary crystallite size weresynthesized from the polycondensation of charged complexspecies containing Cu+, Mn+, Sn4+, CS(NH2)2, S2 and OHin various solvents such as ethylene glycol, ethanol andisopropanol. A variety of ultrafine precursors was synthesizedconsisting of 150–250 nm discrete, loose packed aggregates(Fig. 1) and displaying surface charge. Although all thesea CIRIMAT, CNRS, INPT, Universite´ de Toulouse, 118,Route de Narbonne, 31062 Toulouse Cedex 9bLPCNO, INSA, CNRS, Universite´ de Toulouse, 135,Avenue de Rangueil, 31077 Toulouse Cedexprecursors display a single phase as shown from XRD andRaman spectroscopy, slight differences in relative proportionsin Raman peak surfaces located in the region 200–400 cm1and assigned to secondary and main Cu2MSnS4 vibrations13were observed. Further investigations are in progress to under-stand the origin of these short-range order differences. In atypical experiment, Cu2CoSnS4 ultrafine precursor wassynthesized using the following molar ratios: 2.0 CuCl22H2O : 1.0 CoCl26H2O : 1.0 SnCl45H2O : 10 CS(NH2)2 : 1.0TMAOH. The resulting mixture was transferred into aTeflon-line stainless steel autoclave for 16 h at 200 1C. Theprecipitated solid was washed and ball milled in EtOH using0.3 mm ZrO2 balls. After re-dispersion in EtOH, the ultrafineprecursor was collected after size selection carried out bycentrifugation at 3000 rpm. The particles were aggregates witha mean diameter of 150 nm (Fig. 1) and formed from primarycrystallites of 6 nm average size, as calculated from Scherrer’sequation using the full width at half maximum (FWHM) ofthe (112) Cu2CoSnS4 peak. In the second stage, the high-temperature growth stage was carried out in molten KSCN at450 1C for 6 h. Because the molten salt acts as a sulfidesource, a crucial parameter to control growth kinetics is theCu2CoSnS4 nanocrystal volume fraction, Fvnano, Fvnano =Vnano/(Vnano + VMS), where Vnano and VMS denote, respec-tively, the volume occupied by the nanocrystals and thevolume occupied by the molten salt. Indeed, FVnano dictatesboth the supersaturation level and the interparticle distance ofthe nanocrystals dispersed in the molten KSCN. High-temperature growth regimes were investigated for temperaturesranging from 300 to 550 1C and with various FVnano values,0.045 o Fvnano o 0.17. Formation of a Cu2CoSnS4 singlephase was achieved up to 450 1C as demonstrated from XRDand Raman spectroscopy investigations. As expected, thehigh-temperature dissolution–reprecipitation process inducesbetter crystallization as evidenced from the FWHM of the(112) Cu2CoSnS4 XRD peak. Moreover, a decrease of theFWHM (11.5 cm1) of the 327 cm1 Raman main vibration(Fig. 2) indicates a significant improvement of the short-rangeorder into the nanocrystals. Indeed, large variations in crystalsize were observed depending on the chemical properties ofthe Cu2CoSnS4 precursors. Hence, nanocrystals were onlyproduced with the use of precursors possessing Raman peaksurface proportions similar to those determined from the bulkCu2CoSnS4 spectrum.Since the various precursors investigated exhibit similar sizedistributions, differences observed on the correspondingcurves describing variation of the crystal size with dissolution–reprecipitation temperatures (Fig. 3) clearly indicate differentdissolution kinetics for the Cu2CoSnS4 precursors. A powerlaw increase of nanocrystal size with dissolution–reprecipitationtemperature was observed for these highly homogeneousprecursors. Our results indicate that the successful preparationof discrete nanocrystals rely on precursor properties such as anultrafine size and a high short-range order as determined fromRaman spectroscopy. A more uniform nanocrystal sizedistribution was achieved with careful control of Fvnano (optimalvalue = 0.06), temperature and time. Typically, Cu2CoSnS4solid precursor was mixed with KSCN (Aldrich, 99.99%) in aweight ratio of 1.00 : 5.00 and ground for 10 mn. The mixturewas then placed within an alumina crucible which wassubsequently heated to 450 1C for 6 h under N2 atmosphere.Fig. 1 TEM micrographs of (a) Cu2CoSnS4 precursors showing150 nm discrete nanoparticles and (b) Cu2ZnSnS4 precursors withmean size of 250 nm.Fig. 2 (a) XRD pattern of highly crystallized Cu2CoSnS4nanocrystals. (b) Raman spectrum of highly crystallized Cu2CoSnS4nanocrystals. Inset: Deconvolution of the 200–400 nm region showinga main peak at 327 cm1. Corresponding XRD pattern and Ramanspectrum of Cu2CoSnS4 precursors are indicated in dashed curves,highlighting the effect of the crystallization stage.Fig. 3 (a) Cu2CoSnS4 precursor Raman spectra. (b) (112)Cu2CoSnS4 XRD peaks plotted as a function of dissolution–reprecipitation temperature. The Raman spectra and XRD peakevolutions correspond to samples displaying after deconvolutiondifferent (left figures a1 and b1) or similar (right figures a2 and b2)peak surface proportions compared with bulk Cu2CoSnS4. (c) Inset:XRD crystal size as a function of dissolution–reprecipitationtemperature.After being cooled at room temperature, samples weresubsequently washed with deionized water and ethanol.Transmission electron microscopy (TEM) images of theas-synthesized materials prepared using an ultrafine precursorshow a uniform distribution of nanocrystals (Fig. 4). Theparticles were faceted and have a mean size as small as30 5 nm. From an XRD investigation and using the Scherrerformula, the size of the primary crystallites were determined tobe 30  2 nm, demonstrating that our nanocrystals are singlecrystals. Unlike previous quaternary chalcopyrite nanocrystalsreported in the literature,6 UV-Vis absorption spectrarecorded on the Cu2CoZnS4 nanocrystals synthesized at hightemperature exhibit a distinct excitonic peak observed at830 nm (Eg = 1.5 eV). The observation of a well-definedabsorption peak is the signature of both the high crystallinequality and the homogeneous composition of our nanocrystals.The adaptability of the synthetic method was shown for thepreparation of Cu2ZnSnS4 (CZTS) nanocrystals. A family ofCZTS precursors displaying controlled dissolution kinetic wasprepared by varying the solvothermal reaction temperature(150–200 1C) and adjusting the [Cu+] concentration. High-temperature growth regimes for dissolution–reprecipitationreactions were designed in the 400–500 1C temperature rangeand Fvnano = 0.03–0.18, resulting in discrete CZTS nano-crystals with size ranging from 80 to 250 nm as shown byTEM (Fig. 5). The high crystallization of the nanocrystals isconsistent with the features of the Raman spectrum displayinga low FMWH value (6 cm1) of the 335 cm1 main peakassigned to Cu2ZnSnS4.13 Further work involving precursorsexhibiting smaller particle size, aimed towards the fabricationof Cu2ZnSnS4 single crystals, is in progress.In conclusion, a high-temperature process involving thedissolution reprecipitation of Cu2MSnS4 (M = Co, Zn)ultrafine precursors was developed for the preparation ofhighly crystallized quaternary chalcogenide nanocrystals.Successful synthesis of 30 nm Cu2MSnS4 nanocrystals withcontrolled morphology was achieved through the fabricationof tailored precursors exhibiting ultrafine size and controlleddissolution kinetic.We thank Alain Seraphine for very helpful discussions. Thisresearch was partly funded by Ecole des Beaux Arts etd’Architecture-Re´gion la Reunion, France.Notes and references1 (a) A. Luque and A. Marti, Phys. Rev. Lett., 1997, 78, 5014;(b) A. J. Nozik, Phys. E., 2002, 14, 115; (c) R. P. Raffaelle,S. L. Castro, A. F. Hepp and S. G. Bailey, Progr. Photovolt.:Res. Appl., 2002, 10, 433.2 W. U. Huynth, J. J. Dittmer and A. P. Alivisatos, Science, 2002,295, 2425.3 (a) Q. Guo, S. J. Kim, M. Kar, W. N. Shafarman, R. W. Birkmira,E. A. Stach, R. Agrawal and H. W. Hillhouse, Nano Lett., 2008, 8,2982; (b) M. G. Panthani, V. Akhavan, B. Goodfellow,J. P. Schmidtka, L. Dunn, A. Dodabalapur, P. F. Barbara andB. A. Korgel, J. Am. Chem. Soc., 2008, 130, 16770.4 H. W. Schock and U. Rau, Phys. B, 2001, 308–310, 1081.5 (a) H. X. Schock,MRS Bull., 1993, 28(10), 42; (b) M. A. Contreras,B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoonand R. Noufi, Progr. Photovolt.: Res. Appl., 1999, 7, 311.6 (a) O. Guo, H. W. Hillhouse and R. Agrawal, J. Am. Chem. Soc.,2009, 131, 11672; (b) C. Steinhagen, M. G. Panthani, V. Akhavan,B. Goodfellow, B. Koo and B. A. Korgel, J. Am. Chem. Soc., 2009,131, 12554; (c) C. An, K. Tang, G. Shan, C. Wang, L. Huang andY. Qian, Mater. Res. Bull., 2003, 38(5), 823.7 (a) K. Ito and T. Nakazawa, Jpn. J. Appl. Phys., 1988, 27(Part 1,No. 11), 2094; (b) H. Katagiri, K. Jimbo, S. Yamada,Y. Kamimura, W. S. Maw, T. Fukano, T. Ito and T. Motohiro,Appl. Phys. Express, 2008, 1, 041201; (c) T. K. Todorov,K. B. Reuter and D. B. Mitzi, Adv. Mater., 2010, 22, 1.8 S. R. Hall, J. T. Szymanski and J. M. Stewart, Can. Miner., 1978,16, 131.9 A. Shavel, J. Arbiol and A. Cabot, J. Am. Chem. Soc., 2010, 132,4514.10 A. Eluard and B. Tremillon, J. Electroanal. Chem., 1967, 13, 208.11 P. Boilac, V. Tanner and A. Garumann, Cryst. Res. Technol., 1982,17(6), 717.12 (a) J. Chen, X. Xing, A. Watson, W. Wang, R. Yu, J. Deng,L. Yan, S. Ce and Chen, Chem. Mater., 2007, 19(15), 3598;(b) H. Zhou, Y. Mao and S. S. Wong, J. Mater. Chem., 2007,17, 1707.13 M. Himmrich and H. Haeusler, Spectrochim. Acta, Part A, 1991,47, 933.Fig. 4 Highly crystallized Cu2CoSnS4 nanocrystals. (a) TEM imageshowing discrete well faceted nanocrystals with a mean size of 30 nm.(b) UV-Vis absorption curve exhibiting a excitonic peak at 830 nm(1.5 eV).Fig. 5 Highly crystallized Cu2ZnSnS4 nanocrystals. (a) XRD patternexhibiting a Cu2ZnSnS4 single phase. (b) Raman spectrum showing amajor vibration at 335 cm1. Note the decrease of the FWHM of this335 cm1 peak compared with precursor (dashed curve) demonstratingan increase of the short range order of the nanocrystals. (c) TEMimages showing discrete nanocrystals with well faceted shapes and amean size around 150 mn.

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Highly-crystallized quaternary chalcopyrite nanocrystals via a high-temperature dissolution–reprecipitation route

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