7316 Chem. Commun., 2013, 49, 7316--7318 This journal is c The Royal Society of Chemistry 2013

Cite this: Chem. Commun.,2013,49, 7316

Precursor driven one pot synthesis of wurtzite andchalcopyrite CuFeS2†

Prashant Kumar, Sitharaman Uma and Rajamani Nagarajan*

A facile precursor dependent single step, one pot solution based

synthesis of wurtzite and chalcopyrite polymorphs of CuFeS2 has been

developed by reacting a Cu(I) thiourea complex with Fe2(SO4)3 and

FeCl3 separately in ethylene glycol. The phases have been characterized

by structural refinements, SEM-EDX, TEM-SAED, Raman, UV-Visible

spectroscopy and TGA measurements.

Polymorphism and the structure–property relationship of solids arequite synonymous in chemical research and play a pivotal role indetermining their eventual use for any particular application.1 Cu–Ssystem represents a gallery of stoichiometric and non-stoichiometriccompositions exhibiting polymorphism.2 Complex crystal structure,the highly mobile nature of Cu and the existence of mixed valencestates in copper sulfides2,3 offer chemical and structural freedom forthe easy diffusion of other metal ion/ions in them resulting internary and quaternary sulfides. With the advent of nanoscience andnanotechnology, stabilization of high energy phases, wurtzite andzinc blende, which are the structural variants of generic chalcopyritestructures, by solution based methods has been achieved for theI–III–VI2 type compounds, where group I and III elements belong tocoinage metals and p-block elements of the periodic table, respec-tively, e.g. CuInS2, CuGaS2.4,5 Generation of such polymorphs hasbeen quite beneficial to tune the Fermi energy of these photovoltaicmaterials over a wide range during their fabrication.6 To the best ofour knowledge, there exists no report on the existence of wurtzitestructure with a group III element belonging to the transition metalseries. Presence of transition metal ions in wurtzite arrangementmay evolve interesting properties (as magnetic semiconductors forspintronics) due to the introduction of d orbitals with unpaired

electrons in the band structure.6 Chalcopyrite (CH–CuFeS2) is theextensively studied system among the ternary compositions contain-ing transition metal ions and its structure consists of Cu1+ and Fe3+

at alternate tetrahedral sites of a cubic close packed sulfur networkin tetrahedral symmetry (space group I%42d).7 In this communication,we report a rapid, one pot, single step solution based synthesis ofCuFeS2 in wurtzite (WZ) structure for the first time. Additionally,CuFeS2 in the well known chalcopyrite structure has been synthe-sized by varying the precursor. In the present set of reactions, theCu(I) complex, ([Cu4(tu)9](NO3)4�4H2O (tu = thiourea)), was refluxedwith iron(III) salts, Fe2(SO4)3, FeCl3 separately in ethylene glycol.Thiourea, present in the complex, acted as the sulfur source.

The powder X-ray diffraction (PXRD) pattern of the productobtained from the reaction of [Cu4(tu)9](NO3)4�4H2O with Fe2(SO4)3

is reproduced in Fig. S1 (ESI†). The search-match procedure for theobserved peak positions with the known compositions in the ICSDdatabase using the High score plus software8a did not provide anysolution resembling the fingerprints of binary and ternary sulfidescontaining copper and/or iron. Initially, the sample was subjectedto elemental color mapping of its Field Emission Scanning ElectronMicroscopy (FE-SEM) images to determine the elements present,their homogeneity, purity, phase formation, and the results arereproduced in Fig. 1(a). From the mapping, the product has beenfound to contain Cu, Fe and S elements that are homogeneouslydistributed within the particles. The ratio of Cu : Fe : S obtainedfrom the EDX analysis is 1.16 : 1 : 1.80 (Fig. S2, ESI†). By wetchemical analysis and atomic absorption spectroscopy measure-ments, a similar ratio of the three constituents has been realized(details are provided in ESI†). Preliminary indexing of the PXRDpattern using the TREOR8b program indicated that all the reflec-tions could be indexed in a hexagonal system. Additionally, it wasrecognized that the intensity pattern of the reflections matchedclosely with the observed PXRD pattern of WZ–CuInS2.4 The crystalstructure analysis of the product was therefore carried out byconsidering the structural model of wurtzite CuInS2,9 wherein theIn3+ site is replaced by Fe3+-ions and positional refinement forcopper and iron was carried out using the FULLPROF10 program.The diffraction pattern fitted well in the hexagonal symmetry(space group P63mc (186)) with lattice constants of a = 3.726 (3) Å

Materials Chemistry Group, Department of Chemistry, University of Delhi,

Delhi 110007, India. E-mail: rnagarajan@chemistry.du.ac.in;

Fax: +91-11-2766 6605; Tel: +91-11-2766 2650

† Electronic supplementary information (ESI) available: Synthesis and character-ization of [Cu4(tu)9](NO3)4�4H2O and CuFeS2, PXRD patterns and EDX analysis ofWZ–CuFeS2 and CH–CuFeS2, Rietveld refinement plots of WZ- and CH–CuFeS2,crystallographic data of WZ- and CH–CuFeS2, TEM images of WZ- and CH–CuFeS2, TGA traces of WZ- and CH–CuFeS2 in a nitrogen atmosphere and the PLspectrum of CH–CuFeS2 obtained upon exciting at l = 500 nm. See DOI: 10.1039/c3cc43456g

Received 9th May 2013,Accepted 19th June 2013

DOI: 10.1039/c3cc43456g

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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 7316--7318 7317

and c = 6.132 (4) Å (Fig. 1(b)) after automatic background correction.The structural positional parameters after the final cycle of refine-ment are summarized in ESI† (Fig. S3 and Tables S1 and S2). Theestimated average crystallite size of this sample by Scherrer analysisis 17 nm. Crystal structure of the synthesized WZ–CuFeS2, generatedusing the DIAMOND8c software using the refined parameters, ispresented in Fig. 1(c). The Selected Area Electron Diffraction (SAED)pattern of this sample is shown in Fig. 1(d); the diffraction spotscorresponding to the (002), (110), (012)/(102), (011)/(101) planesmatch well with its PXRD pattern. The TEM image of the sampleis shown in Fig. S4 (ESI†).

WZ–CuFeS2 nanocrystals show micro flower-like morphologywith an average diameter of 0.8–1 mm (Fig. 2(a)). It shows fourbands at 287, 351, 470 and 564 cm�1 in the room temperatureRaman spectrum (Fig. 2(b)). While the positions of the bands matchwell with the ones reported for CH–CuFeS2,11 the intensity of theband at 470 cm�1 is slightly lower than that of the band at 287 cm�1.Following the assignment of these modes for CH–CuFeS2,11 theobserved bands in the present case have been assigned to A1, B2 andE phonon modes of the lattice. The band at 564 cm�1 may be thefirst overtone of 287 cm�1. The observed intensity changes in the A1

and E modes may arise from internal changes in the dipolemoments in the hexagonal lattice consisting of Cu1+ and Fe3+ ions.Fig. 2(c) presents the UV-Visible absorption spectrum of the synthe-sized WZ–CuFeS2, dispersed in n-hexane. CuFeS2 is known to show aligand to metal charge transfer spectrum due to the presence ofempty d orbitals in Fe3+. Prominent absorption occurring in therange of 500–780 nm confirms the charge transfer in this system.12

The optical band gap of WZ–CuFeS2 has been estimated to be 0.7 eV(inset in Fig. 2(c)). A strong emission centered at 750 nm is observedin the photoluminescence (PL) spectrum upon exciting the sampleat l = 500 nm (Fig. 2(d)). Although this kind of emission has beenattributed to the luminescence from donor to acceptor defect levelsin WZ–CuInS2 and WZ–CuGaS2,13 intraband optical transitions fromCu1+ present in CuFeS2 cannot be ruled out.11

Upon reacting FeCl3 with the same Cu–tu precursor, for-mation of tetragonal CuFeS2 (JCPDS File no. 83-0983) is evidentfrom its PXRD pattern (Fig. 3(a)). Lattice dimensions obtainedfrom the Rietveld refinement in space group I%42d (122) area = 5.386 (5) Å and c = 10.391 (1) Å (Fig. S5 and Tables S3 and S4,ESI†). The average crystallite size has been estimated to be10 nm by the Scherrer analysis. Similar to the wurtzite phase, aflower-like morphology comprising of plate shaped petals hasbeen noticed for CH–CuFeS2 in its FESEM images (Fig. 3(b)).EDX analysis confirmed the presence of Cu, Fe and S yielding aratio of 1.25 : 1.08 : 1.70 (Fig. S6, ESI†). Rietveld refinements ofCuFeS2 in WZ and CH structures have been compared in Fig. S7(ESI†). The SAED-TEM pattern of this sample (shown in theinset of Fig. 3(b) and Fig. S8, ESI†) endorses the tetragonalsymmetry by the presence of spots corresponding to (112) and(220) planes. The room temperature Raman spectrum presentedin Fig. 3(c) also confirms the chalcopyrite structure exhibitingbands at 287, 351 and 470 cm�1 corresponding to the A1, B2 andE phonon modes respectively.11 The UV-Visible spectrum of theCH–CuFeS2 sample is shown in Fig. 3(d) and the calculatedband gap of 0.6 eV (inset in Fig. 3(d)) is quite comparable to thereported range of 0.53–0.7 eV.12 The observed band gap ofthe CH–CuFeS2 is marginally smaller than that of its WZcounterpart and such trends have been reported earlier forternary and quaternary sulfides based on ZnS structure.14,15

The temperature dependent weight loss pattern of WZ–CuFeS2

differs slightly from that of CH–CuFeS2 as shown by theirthermograms in a nitrogen atmosphere (Fig. S9, ESI†). A PLemission spectrum similar to the one observed for WZ–CuFeS2

has been obtained for CH–CuFeS2 samples upon excitation atl = 500 nm (Fig. S10, ESI†).

Solution based synthesis of CH–CuFeS2 was first reported byRoberts16 who reacted CuS and FeS precipitates under mildconditions. Earlier studies on the solution based synthesis ofI–III–VI2 including CuFeS2 have been centred mainly on thegeneration of different shapes and sizes of the ternary sulfidesin which the change in metal ion precursors in a single solvent

Fig. 1 (a) Elemental distribution of S, Fe and Cu in the samples, (b) Rietveld fit ofthe powder X-ray diffraction (PXRD) pattern (observed, calculated (profile matching),and difference profiles are given as red, blue, and green lines, respectively) of theproduct obtained from the reaction of [Cu4(tu)9](NO3)4�4H2O with Fe2(SO4)3�xH2O inethylene glycol for 1.5 h under refluxing conditions, (c) structure of wurtzite form ofCuFeS2, and (d) SAED pattern of wurtzite CuFeS2.

Fig. 2 (a) FE-SEM images, (b) room temperature Raman spectrum, (c) UV-Visibleabsorption spectrum with the inset of the energy (eV) vs. (Ahn)2 plot of theoptical absorption data, and (d) photoluminescence spectrum of WZ–CuFeS2

obtained by exciting at l = 500 nm.

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7318 Chem. Commun., 2013, 49, 7316--7318 This journal is c The Royal Society of Chemistry 2013

system did not result in any symmetry variation of the finalproduct.17 Our study is the first report in which the change insymmetry of the ternary sulfide has been achieved underidentical conditions just by changing the Fe3+ salt using asingle solvent system, viz., ethylene glycol. It is believed thatthe less coordinating ability of ethylene glycol promotes thehomogenous reaction between the reactants effectively pavingthe way for the rapid inclusion of Fe3+ in the Cu–S lattice.

Among the plausible mechanisms responsible for the formationof CuFeS2 in hexagonal and tetrahedral symmetries, simultaneousnucleation of hexagonal CuxS and hexagonal FexS crystals and theirinterdiffusion may occur during the dissociation of the Cu–tuprecursor in the presence of Fe2(SO4)3. Such a proposition wouldrequire minimum energy for the rapid lattice formation due tosymmetry match. Along the similar lines, reaction of tetragonalCuxS nuclei with tetragonal FexS nuclei resulting in tetragonalCuFeS2 might proceed when FeCl3 is employed. It is relevant topoint out that incorporation of 0.05% and 5% Fe3+ has stabilizedthe monoclinic and tetragonal symmetries of Cu2S.18 Neverthelessthe dramatic change in the symmetry of CuFeS2 by just changing theanion of the iron salt justifies the need to understand the intriguingchemistry in these systems.

In summary, a simple non-injection synthetic approach togenerate CuFeS2 in WZ and CH structures by reacting differentFe3+ salts with the same Cu–tu precursor in ethylene glycol hasbeen developed. This process is viable and scalable as quantitativeyields (average yield of 80%) of nano sized powders that are readilyredispersable in non-polar solvents are obtained. The discovery of anew phase (WZ–CuFeS2) will be of immediate significance to tunethe optoelectronic and magnetic properties.

The authors thank DST (Nanomission) and DST (SR/S1/PC-07/2011(G)), New Delhi, Govt of India, for the financial supportto carry out this work.

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8 (a) X’Pert High Score Plus, PANalytical, Almeo, The Netherlands,2004; (b) P.-E. Werner, L. Eiksson and M. Westdahj, Appl. Crystal-logr., 1985, 18, 367–370; (c) K. Brandenburg and H. Putz, Diamond,Version 3.0, Germany, 2005.

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Fig. 3 (a) Rietveld fit of the PXRD pattern along with the JCPDS file of CH–CuFeS2

(observed, calculated (profile matching), and difference profiles are given as red,blue, and green lines, respectively), (b) FE-SEM image with the SAED pattern in theinset, (c) room temperature Raman spectrum, (d) UV-Visible absorption spectrumof the product obtained from the reaction of [Cu4(tu)9](NO3)4 with FeCl3 inethylene glycol for 1.5 h under refluxing conditions. The inset in (d) shows theplot of energy (eV) vs. (Ahn)2 of the optical absorption data.

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