SEMI-INDUSTRIAL GREEN MECHANOCHEMICAL SYNTHESES OF

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 2132−2141

pubs.acs.org/journal/ascecg

Semi-industrial Green Mechanochemical Syntheses of Solar Cell Absorbers Based on Quaternary Sulfides Peter Baláz,̌ † Michal Hegedüs,*,‡ Marcela Achimovičová,†,∥ Matej Baláz,̌ † Matej Tešinský,† Erika Dutková,† Mária Kaňuchová,§ and Jaroslav Briančin† †

Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, 04001 Košice, Slovakia Institute of Chemistry, Faculty of Science, P. J. Šafárik University in Košice, Moyzesova 11, 04001 Košice, Slovakia § Institute of Earth Resources, Faculty of Mining, Ecology, Process control and Geotechnologies, Technical University in Košice, Letná 9, 04001 Košice, Slovakia ∥ Institute of Mineral and Waste Processing, Waste Disposal and Geomechanics, University of Technology, 38678 Clausthal, Germany ‡

ABSTRACT: Stannite Cu2FeSnS4 and kesterite Cu2ZnSnS4 were synthesized by an eco-friendly ball-milling method in an industrial mill. Both synthesized sulfides represent perspective materials in solar cell technology. Several characterization methods have been applied to determine the course of solid state mechanochemical reactions leading to the synthesis of potential chalcogenide solar cell absorbers. The phase and surface composition, solid state kinetics, and surface morphology of these quaternary sulfides were elucidated by the methods of X-ray diffractometry, X-ray photoelectron spectroscopy, Soxhlet analysis, and scanning electron microscopy. The application of eccentric vibration mills for these syntheses constitutes the big challenge for researchers in the field of photovoltaics in their permanent effort in scaling up new materials and processes. KEYWORDS: Solid-state reactions, Mechanochemistry, Scalable synthesis, Advanced materials, Photovoltaics, Stannite, Kesterite

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medicinal chemistry can serve as a good illustration.11−24 In energy-related topics such as solar cells and thermoelectric materials this trend is also strongly visible. Kesterite Cu2ZnSnS4, stannite Cu2FeSnS4, and other quaternary sulfides have recently proved to be effective on a laboratory scale according to several papers.25−31 However, this eco-friendly and solvent-free synthesis approach needs validation for production of materials in scalable amounts. Vibration mills and their industrially successful alternative, eccentric vibration mills (ESMs), represent a suitable option of high energy mills. Theory and practical application of ESMs are closely bound with the works of Professor Gock and his co-workers.32−36 The ESM mills have modular tube design in which the exciter unit consisting of a bearing block and unballance masses are connected to the motor by means of the drive shaft. For the purpose of mass compensation, a counterweight is located on the opposite side to the exciter unit. The major technological advance is the fixed amplitude of vibration up to 20 mm (normally 12 mm maximum) and eliptical, circular, and linear vibrations in comparison with circular vibrations in conventional mills, except for “traditional” applications in mineral processing, hydrometallurgy, soil rehabilitation, pharmaceutical industry, and agriculture, etc. where ESMs were applied for

INTRODUCTION Solar energy represents the best alternative as an eco-friendly and inexhaustible clean energy in comparison with the energy obtained from traditional exhaustible fossil fuels. Until now, the effort to gain electrical energy from current photovoltaic panels was more expensive in comparison with traditional thermal treatment of fossil fuels. The effort to optimize the solar materials is therefore urgent.1−8 Among the quaternary chalcogenides Cu(Inx,Ga1−x)(S,Se)2 (CIGS) thin films represent an interesting option, as in their case, more than 20% efficiency is achieved.9 However, the main drawbacks of the CIGS are Se toxicity, In and Ga economy, and their abundance. Therefore, an effort to find analogous materials must be backed-up by the selection of eco-friendly and inexpensive precursors for synthesis. Stannite Cu2FeSnS4 and wellelaborated kesterite Cu2ZnSnS4 with their promising photoelectric properties represent alternatives to CIGS. The latest method for the CIGS preparation follows a twostep approach comprising a low temperature precursor deposition and subsequent high temperature selenization/ sulfidization annealing.10 However, several excellent papers were published recently in which application of high-energy milling has been described as an effective way for preparation of new compounds via solid-state synthesis. This laboratory approach can produce advanced materials within the frame of inorganic and organic chemistry. The application in material science, heterogeneous catalysis and pharmaceutical and © 2018 American Chemical Society

Received: October 3, 2017 Revised: December 11, 2017 Published: January 10, 2018 2132

DOI: 10.1021/acssuschemeng.7b03563 ACS Sustainable Chem. Eng. 2018, 6, 2132−2141

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Eccentric vibratory mill with attached closed satellite (left), open satellite filled with balls (right). nm in quartz cell by dispersing the synthesized particles in absolute ethanol by ultrasonic stirring for 10 min. Scanning Electron Microscopy (SEM). SEM images of the samples were recorded using MIRA3 FE-SEM microscope (TESCAN, Czech Republic) equipped with an EDX detector (Oxford Instrument, United Kingdom). Soxhlet Analysis. The content of nonreacted elemental sulfur in the samples was determined by a gravimetric method using a Soxhlet extractor. The extraction thimble was loaded with 1 g of powder sample and placed into the main chamber of the extractor. A 30 mL aliquot of carbon disulfide (99.9%, Fischer Chemical) was placed into a 500 mL distilling flask, and the apparatus was set up. Upon heating of the distilling flask in a hot water bath, three cycles were run to quantitatively dissolve all sulfur present in the sample. The resulting solution cooled to room temperature was transferred into a 250 mL round-bottom flask of a known mass, and the solvent was distilled under vacuum. The flask was further dried at 70 °C to eliminate any traces of the solvent and reweighed. Conversion degree α of the corresponding mechanochemical synthesis was calculated based on Soxhlet analysis according to the formula

mechanical activation of solids, they could be also used in “advanced” applications as mechanochemical reactors to produce new compounds. In this case, solid state reactions are executed directly in the mill. The aim of this paper is to illustrate effectiveness of an industrial eccentric vibration mill for mechanochemical synthesis of two quaternary sulfides suitable as perspective absorbers in solar cells.

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MATERIALS AND METHODS

Materials. For mechanochemical synthesis various precursors were applied. Copper (99% Merck, Germany), iron (99% Winlab, Germany), zinc (99% Aldrich, Germany), tin (99% Nihon Seiko, Japan), and sulfur (99% CG-Chemikalien, Germany) powders were used as elemental precursors. Natural CuFeS2 (98%, deposit Zhezkazgan, Kazakhstan, JCPDS 00-035-0752), mechanochemically synthesized copper sulfide CuS (JCPDS 00-006-0464) and tin sulfide SnS (JCPDS 00-039-0354) were used as precursors. Mechanochemical Synthesis. Mechanochemical solid state syntheses were performed in an industrial eccentric vibratory ball mill ESM 656−0.5 ks (Siebtechnik, Germany) working under the following conditions: A 5 L steel satellite milling chamber attached to the main corpus of the mill, tungsten carbide balls with a diameter of 35 mm with a total mass of 30 kg, 80% ball filling, amplitude of the mill = 20 mm, rotational speed of the eccenter = 960 min−1, argon atmosphere, 100 g of feed. The milling was performed at various times. In the case of mechanochemical synthesis of CuS and SnS precursors, milling for 40 and 60 min, respectively, has been applied. The photographs of the mill with satellite are given in Figure 1. Stoichiometric ratios of precursors have been applied according to reactions given in the Results and Discussion section. Characterization Techniques. X-ray Diffractometry (XRD). The identification of phase composition was performed by XRD method with an X’Pert PW 3040 MPD diffractometer (Phillips, Germany) working in the 2θ geometry with Cu Kα radiation. Photoelectron Spectroscopy (XPS). XPS measurements of samples were performed using XPS instrument SPECS equipped with PHOIBOS 100 SCD and nonmonochromatic X-ray source. The survey surface spectrum was measured at 70 eV pass energy and the core spectra was measured at 20 eV at room temperature. All spectra were acquired at a basic pressure of 1.10−8 mbar with Al Kα excitation at 10 kV (200 W). The data were analyzed by SpecsLab2 CasaXPS software (Casa Software Ltd.). A Shirley and Tougaard type baseline was used for all peak-fits. The spectrometer was calibrated against silver (Ag 3d). All samples showed variable degree of charging. The problem was resolved by the calibration on carbon. UV−vis Spectroscopy (UV−vis). UV−vis spectra were collected using the UV−vis spectrophotometer Helios Gamma (Thermo Electron Corporation, Great Britain) working in the range 200−900

α=1−

(mF1 − mF0) mS0

(1)

where α is overall conversion degree, mso is the starting amount of sulfur in the reaction mixture (g), mf1 is mass of the flask after extraction (g), and mf0 is mass of the flask before extraction (g).

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RESULTS AND DISCUSSION In this part we evaluate various mechanochemical pathways to synthesize stannite Cu2FeSnS4 and kesterite Cu2ZnSnS4 in an industrial scale for solar cell absorbers. Methods of XRD (phase analysis), XPS (surface composition), SEM (morphology), Soxhlet analysis (kinetics), and UV−vis (band gap calculation) were applied as the main identification and characterization tools. Stannite Cu2FeSnS4. Synthesis from Copper Mineral and Synthetic Compounds. Chalcopyrite CuFeS2 mineral in combination with tin sulfide SnS and copper sulfide CuS compounds were applied for mechanochemical synthesis of stannite. The overall hypothetical equation of the synthesis can be proposed in the form CuFeS2 + CuS + SnS → Cu 2FeSnS4

(2)

The reaction mixture corresponding to the stoichiometry of eq 2 was milled for 60, 120, and 240 min, respectively. X-ray diffractograms are given in Figure 2. All XRD patterns are strongly influenced by amorphization and formation of small crystallites. Stannite Cu2FeSnS4 (JCPDS 00-044-1476) is 2133


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the region 932.0−932.4 eV is also typical for Cu2+ in CuS38 which is also well suited for the first peak in Figure 3A. The surface iron is represented by peaks at 711.06 eV, 715.66 eV, and 724.99 eV (Figure 3B). The spectrum is in good agreement with the values for Fe2+ ion with the binding energy of 710.3 eV and 723.5 eV, respectively.39 Peaks at 494.50 eV and 503.05 eV and a peak splitting of 8.6 eV in Figure 3C indicate the presence of Sn4+ ion in the structure.40 The peak at 161.85 eV (Figure 3D) is consistent with the values 160−164 eV expected for S2− in sulfide phases.41 Figure 4 shows SEM images for low and high magnification of samples milled for 240 min. It is evident that the sample is formed by particles of different sizes where tendency to form microagglomerates can be seen (Figure 4A). By larger magnification, submicrometer particles can be traced. These are also agglomerates composed of nanoparticles in the form of plates (Figure 4B). Synthesis from Synthetic Compounds and Elements. Compounds tin sulfide SnS and copper sulfide CuS in combination with elements Fe and S were applied for mechanochemical synthesis of stannites. The overall hypothetical equation of the synthesis can be proposed as the form

Figure 2. XRD patterns of the powders milled according to eq 2 for different times.

identified already at the shortest milling time (60 min), Figure 2A. However, its identification is complicated because the main peak is overlapped by other peaks of unreacted chalcopyrite CuFeS2 (JCPDS 00-035-0752) and covellite CuS (JCPDS 00024-0060). Unreacted herzenbergite SnS (JCPDS 00-0140620) is also present. The further progress of reaction 2 is manifested by more diffused XRD peaks and the absence of SnS in the XRD spectra. However, rhodostannite Cu2FeSn3S8 (JCPDS 00-029-0558) as further stannite phase can be identified by milling for 240 min. The mechanism of the new phase formation is even more complicated, because already at 120 min the additional quaternary sulfide masonite Cu6Fe2SnS8 (JCPDS 00-029-0557) starts to be present. X-ray photoelectron spectroscopy was used to elucidate the surface composition of as-synthesized powders. High resolution XPS spectra are shown in Figure 3. Figure 3A shows the most intensive copper binding peaks at 932.10 and 951.70 eV with a peak splitting of 19.8 eV. Undoubtedly, the splitting confirms Cu+ oxidation state in nonconsumed chalcopyrite.37 However,

2CuS + SnS + Fe + S → Cu 2FeSnS4

(3)

The reaction mixture corresponding to the stoichiometry of eq 3 was milled for 30, 60, 120, 240, and 480 min, respectively. Xray diffractograms for all milled samples are shown in Figure 5. They illustrate progress of reaction 3. At the shortest milling times only reaction precursors can be traced (Figure 5A,B). At 120 min of milling the picture is changed: the pattern is more simple and the product stannite Cu2FeSnS4 (JCPDS 00-0351351) is already seen for the first time (Figure 5C). However, traces of covellite CuS (JCPDS 00-039-0354) and herzenbergite SnS (JCPDS 00-0039-0354) are still present. Nonconsumed elemental iron used as precursor in reaction 3 is clearly seen. There is not a big difference between Figure 5C and Figure 5D showing XRD patterns for powders milled for

Figure 3. High-resolution XPS spectra of the powder milled according to eq 2 for 240 min: (A) copper, (B) iron, (C) tin, (D) sulfur. 2134


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Figure 4. SEM images of the powder milled according to eq 2 for 240 min.

Figure 5. XRD patterns of the powders milled according to eq 3 for different times. Figure 6. Conversion degree α vs milling time for stannite synthesis (eq 3).

240 and 480 min. However, in both cases, rhodostannite Cu2FeSn3S8 (JCPDS 00-021-0878) is already present as a second stannite phase. The XRD patterns presented in Figure 5C,D are strongly influenced by amorphization as a consequence of prolonged milling. By mutual comparison of Figure 5C,D it can be stated that the products are more pronounced (see the most intensive peak) and nonreacted Fe is present in a small amount only. Equation 1 has been applied to calculate the conversion degree, α of stannite formation by milling. In Figure 6, the corresponding plot is given. Very high conversion degrees (over 94%) can be obtained at 120 min and longer milling times. This quantitative statement is in a good accordance with qualitative data obtained from XRD measurements. High resolution XPS spectra are given in Figure 7. Figure 7A shows two most intensive copper binding peaks at 932.74 and 952.54 eV. There are also intermittent peaks at 937.07 eV and 944.42 eV. Undoubtedly, the most intensive peaks and a peak splitting of 19.8 eV belong to Cu+ ion configuration in the stannite structure42,43 as in Figure 3A. Two smaller peaks correspond to the Cu2+ ion configuration which forms satellites in the region 930−950 eV.44 The surface iron is represented by peaks at 710.79 eV, 715.09 eV, and 724.82 eV (Figure 7B). The spectrum is in a good agreement with the values for Fe2+ ion with a binding energy of 710.3 eV and 723.5 eV for Fe 2p

levels.39 Peaks at 486.50 eV and 495.23 eV and a peak splitting of 8.7 eV in Figure 7C indicate the presence of Sn4+.40 However, the first peak could be also assigned to Sn2+ configuration from nonconsumed SnS. A peak at 161.46 eV in Figure 7D is consistent with the span of values 160−164 eV expected for S2− configuration in stannite phases.41 SEM images of sample milled for 480 min are shown in Figure 8. The morphology reflects a combination of micro- and nanoparticles with tendency to form agglomerates (Figure 8A). This tendency can be identified by higher magnification also for flat nanoparticles (Figure 8B). Synthesis from Elements. Elements Cu, Fe, Sn, and S were applied for mechanochemical synthesis of stannites. The overall hypothetical equation of the synthesis is proposed as 2Cu + Sn + Fe + 4S → Cu 2FeSnS4

(4)

The reaction mixture corresponding to the stoichiometry of eq 4 was milled for 60, 120, and 240 min, respectively. XRD patterns are given in Figure 9. Already at the shortest milling time (Figure 9A) stannite Cu2FeSnS4 (JCPDS 00-044-1476) is present as a product of reaction 4. However, also other present phases such as copper sulfide CuS (JCPDS 00-006-0464), iron sulfide FeS (JCPDS 00-023-1123) and nonconsumed Fe 2135


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Figure 7. High resolution XPS spectra of the powder milled according to eq 3 for 480 min: (A) copper, (B) iron, (C) tin, (D) sulfur.


(JCPDS 00-004-0673) complete the whole picture. The presence of binary sulfides CuS and FeS confirms that the mechanism of stannite formation proceeds through the intermediate step. The presence of spionkopite Cu1.39S (JCPDS 00-036-0380) as an intermediate is questionable but can be connected with nonstoichiometry in the Cu−S system.45 After prolonged milling for 120 min (Figure 9B) the XRD pattern is more simplified, and only stannite phases can be detected. No impurity peaks from the reaction precursors were observed in the diffraction pattern. Rhodostannite Cu2FeSn3S8 is formed as a product of stannite transformation. There is no difference for this pattern and a pattern of sample milled for 240 min (Figure 9C). Stannite to rhodostannite transformation during milling represents a new phenomenon which was studied in a more detail in our latest work.31

X-ray photoelectron spectroscopy was used to elucidate the surface composition of as synthesized powders. High resolution XPS spectra are given in Figure 10. Figure 10A shows the two most intensive copper binding peaks at 932.74 eV and 952.54 eV. There is also an intermittent coupling between these two peaks. Again, the most intensive peaks and a peak splitting of 19.8 eV belongs to Cu+ ion configuration in the stannite structure.42,46 Two peaks with lower intensity correspond to Cu2+ ion configuration which forms satellites in the region 930−950 eV.44 The surface iron (Figure 10B) is represented by peaks at 711.68 eV and 725.16 eV. Their splitting of 13.5 eV indicates the presence of an Fe2+ ion.39 The peak at 716.83 eV belongs to the Sn2+ configuration. Peaks at 486.50 eV and 495.23 eV with a peak splitting of 8.7 eV in Figure 10C indicate the presence of Sn4+.40 The peak at 161.46 eV in Figure 10D is 2136


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Kesterite Cu2ZnSnS4. Synthesis from Elements. Until now, mechanochemical synthesis of kesterite Cu2ZnSnS4 has been studied using elements and various compounds as precursors in a laboratory mill. Mainly elements and sulfides were applied.47−50 In our case, elements Cu, Zn, Sn, and S were applied for mechanochemical synthesis in an semi-industrial mode. The overall hypothetical equation of the synthesis can be proposed as 2Cu + Zn + Sn + 4S → Cu 2ZnSnS4

(5)

The reaction mixture corresponding to the stoichiometry of eq 5 was milled for 60, 120, and 240 min, respectively. Corresponding XRD patterns are given in Figure 13. Already at the shortest milling time (Figure 13A) kesterite Cu2ZnSnS4 (JCPDS 00-026-0575) is present as a product of reaction 5. However, there is a plenty of other peaks in the XRD pattern which do not correspond to the kesterite phase. Nonreacted Sn (JCPDS 00-004-0673), Zn (JCPDS 00-004-0831) and covellite CuS (JCPDS 00-006-0464) complete the X-ray diffractogram. As in the previous case, the presence of CuS confirms a hypothesis that the mechanism of kesterite formation proceeds through an intermediate step where binary precursors susceptible for synthesis via milling are being formed first. The high reactivity of mechanochemical synthesis of CuS from elemental precursors has been verified recently.51 After prolonged milling for 120 min (Figure 13B), the XRD pattern is simpler. Finally, at a milling time of 240 min (Figure 13C) the kesterite phase is dominant and only a small amount of other phases such as elemental Sn can be traced. Unambiguously, elemetal Zn is still present. Further research is needed to complete reaction 5. According to Park its presence implies that the product can have a Zn-deficient phase formed through the consumption of CuS and elemental Sn.52 High resolution XPS spectra of the mixture milled for 240 min are shown in Figure 14. Figure 14A shows the two most

Figure 9. XRD patterns of the milled according to eq 4 for different times.

consistent with the span of values 160−164 eV expected for the S2− configuration in stannite phases.41 SEM images for the sample milled for 240 min are depicted in Figure 11. The strong tendency to create big agglomerates is evidenced again (Figure 11A) together with preserved nanoparticles (Figure 11B). As for particle size, this is comparable with the size in Figure 8B. The UV−vis spectra of samples milled for 120 and 240 min were studied using absorption UV−vis spectroscopy, and they are given in Figure 12. The as-synthesized samples exhibit a broad optical absorption in the visible wavelength region. The band gap was determined by plotting (Ahν)2 as a function of the photon energy (hν) and extrapolating the linear portion of the spectrum in the band edge region, as shown in the inset of Figure 12. The obtained values of the optical band gap are almost comparable with the previous results reported in the literature.42

Figure 10. High-resolution XPS spectra of the powder milled according to eq 3 for 240 min: (A) copper, (B) iron, (C) tin, (D) sulfur. 2137


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belongs to Cu2+.47 The surface zinc is represented in Figure 14B by peaks at 1021.74 eV and 1034.95 eV. The peak at higher binding energy corresponds to Zn2+.46,53 The second peak belongs to unreacted elemental zinc which can be also traced in the XRD pattern (Figure 13). Peaks at 487.22 eV and 495.79 eV with a separation of 8.6 eV in Figure 14C indicate the presence of Sn4+.53 Peaks at 163.27 eV and a small subpeak at approximately 160 eV in Figure 14D are consistent with the span of values 160−164 eV expected for the S2− configuration in kesterite nanocrystals.54 SEM images of the samples milled according to eq 5 are shown in Figure 15. In comparison with previous SEM pictures for the stannite system (Figures 4, 8, and 11), the tendency to form agglomerates is partly suppressed. However, in the nanorange the particle sizes are comparable and more uniform size distribution can be expected. In confrontation with literature on mechanochemical synthesis of kesterite via milling of elemental precursors,52,53,55−57 common features can be found. The particles are always agglomerated to some extent. However, the careful study of milling time influence on particle morphology has shown that the milled samples illustrated transition from micrometer-sized particles to their submicrometer analogues.52

Figure 12. UV−vis spectra and Tauc plots (the variation of (Ahν)2 vs hν shown in the inset) for samples according to eq 3 milled for 120 and 240 min.

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CONCLUSION In this work stannite Cu2FeSnS4 and kesterite Cu2ZnSnS4 were synthesized in an industrial eccentric vibratory mill. Various experimental approaches were applied for the synthesis using natural and synthetic precursors. Specifically, mineral CuFeS2, synthetic CuS and SnS, as well as Cu, Fe, Zn, Sn, and S elemental precursors were applied for stannite synthesis, and Cu, Zn, Sn, and S for kesterite synthesis. Methods of XRD, XPS, and SEM were used for characterization of bulk, surface, and morphological properties of the synthesized products. The syntheses proceeded via intermediate products among which mainly binary sulfides were identified. The XPS results showed the presence of the elements in anticipated oxidation states and the SEM analysis revealed that all products were present in the form of nanoparticles agglomerated into larger grains. The method of Soxhlet analysis was applied in order to follow the reaction kinetics of stannite formation. For stannite synthesized from elemental precursors, the optical band gap energies were

Figure 13. XRD patterns of the powders milled according to eq 4 for different times.

intensive copper binding peaks at 932.63 eV and 952.23 eV. The peak positions and the peak separation of 19.6 eV indicate the presence of Cu+ configuration in the kesterite structure.47 There is also a weak intermittent peak at 942.09 eV which 2138


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Figure 14. High resolution XPS spectra of the powder milled according to eq 4 for 240 min: (A) copper, (B) zinc, (C) tin, (D) sulfur.


calculated. Particularly, the obtained values of Eg = 1.1−1.2 eV may be well suited for possible application of the synthesized materials as absorbers for solar cells. Generally, good recoveries have been obtained in mechanochemical reactions under study. However, the maintainance of purity of the obtained products still reamins a big challenge. There is a plethora of methods usable for the separation of unreacted species from quaternary sulfides composed of Cu, Fe, Zn, Sn, and S elements. Just to mention a few methods: (i) magnetic separation of nonreacted iron from produced stannite where a big difference in their magnetic susceptibilites can be presumed (ii) selective extraction of nonreacted sulfur via Soxhlet method

(iii) selective dissolution (e.g., FeS is soluble in dilute HCl in contrast to Cu) (iv) thermal post-treatment to remove nonreacted SnS Optimization of milling conditions such as milling time, frequency, and ball-to-powder ratio should be the very first step in mechanochemical synthesis before application of any mentioned methods aimed at the product purification. In optimized milling conditions it is possible that all precursors would be consumed in the course of the reaction. The synthesized quaternary sulfides contain only nontoxic and earth-abundant components. The applied mechanochemical procedure can serve as a new way for low cost scaling in preparation of components for solar cells. 2139


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AUTHOR INFORMATION

Corresponding Author

*Tel.: 421-918-666-136. E-mail: [email protected]. ORCID

Michal Hegedüs: 0000-0003-4037-6208 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Slovak Research and Development Agency (project APVV-0103-14). The work was also performed within the European Community Horizon 2020 Programme, COST action CA15103 (CRM_EXTREME). The support of German Federal Ministry of Education and Research (Project IB-COMSTRUC-010) and the support of Slovak Grant Agency VEGA (projects 2/0044/18 and 2/ 0065) is also gratefully acknowledged.

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