Mechanochemistry of copper sulfides: Characterization, surface oxidation and photocatalytic activity

Accepted Manuscript Mechanochemistry of copper sulfides: Characterization, surface oxidation and photocatalytic activity Matej Baláž, Erika Dutková, Zdenka Bujňáková, Erika Tóthová, Nina G. Kostova, Yordanka Karakirova, Jaroslav Briančin, Mária Kaňuchová PII:

S0925-8388(18)30778-3

DOI:

10.1016/j.jallcom.2018.02.283

Reference:

JALCOM 45159

To appear in:

Journal of Alloys and Compounds

Received Date: 20 July 2017 Revised Date:

21 February 2018

Accepted Date: 23 February 2018

Please cite this article as: M. Baláž, E. Dutková, Z. Bujňáková, E. Tóthová, N.G. Kostova, Y. Karakirova, J. Briančin, Má. Kaňuchová, Mechanochemistry of copper sulfides: Characterization, surface oxidation and photocatalytic activity, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.02.283. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Mechanochemistry of copper sulfides: Characterization, surface oxidation and photocatalytic activity Matej Baláž,1* Erika Dutková,1 Zdenka Bujňáková,1 Erika Tóthová,1 Nina G. Kostova,2 Yordanka Karakirova,2 Jaroslav Briančin,1 Mária Kaňuchová3 1Department

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of Mechanochemistry, Institute of Geotechnics, Slovak Academy of Sciences, Košice, Slovakia 2Institute of Catalysis, Bulgarian Academy of Sciences, Sofia, Bulgaria 3Faculty of Mining, Ecology, Process Control and Geotechnologies, Technical University, Košice, Slovakia

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Abstract

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In the present study, the mechanochemically prepared covellite, CuS and chalcocite, Cu2S nanocrystals are compared in detail. Concretely, the SEM, EDS, N2 adsorption, UV-Vis, PL, FTIR, ZP measurements, PCCS, XPS and DTA/TG were used. SEM, EDS and elemental mapping has shown that the agglomerates containing nanoparticles of the corresponding phases were formed as a result of milling. The nitrogen adsorption method has documented richer surface properties in the case of CuS sample. The specific surface area values were 2.7 m2.g-1 and 1.4 m2.g-1 for CuS and Cu2S, respectively. UV-Vis and PL measurements have shown that these materials might be suitable for optoelectronic applications, as the optical bandgaps were 1.92 eV and 3 eV for CuS and Cu2S, respectively. FTIR spectrum of CuS exhibited the peak at 621 cm-1, which is characteristic for this material. ZP values were more negative in the case of CuS and PCCS has confirmed the finer character of this sample (x50 values were 680 nm and 617 nm for Cu2S and CuS, respectively). XPS method documented the surface oxidation of the prepared Cu2S. The thermal stability measurement up to 550 °C has shown that Cu2S does not undergo significant changes (only 1.4% weight loss was observed), whereas CuS is losing sulfur and is transformed into digenite and chalcocite (14.3% weight loss was observed). The mechanochemically synthesized copper sulfides show high activities in photodecolorization of Methyl Orange dye under visible light irradiation, as Cu2S was able to completely decompose the dye in 150 min and CuS caused 80% decomposition after the same time of treatment. Keywords: copper sulfide; covellite; chalcocite; mechanochemistry; characterization; photocatalytic activity 1. Introduction

Among copper sulfides, covellite, CuS and chalcocite, Cu2S are the most common. These compounds in the form of nanoparticles are extensively studied today, because of their possible utilization in various applications, including biomedical ones [1]. As an example, their application in photothermal ablation can be mentioned [2]. Further applications are as photocatalysts [3, 4], hydrogen gas sensors [5], absorbers for solar cells [6, 7], energy storage devices [8] or thermoelectric materials [9, 10]. There are many synthetic pathways to copper sulfide nanocrystals, like hot injection method [11], hydrothermal [12] and solvothermal synthesis [13], 1

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electrodeposition [14] or microwave irradiation [15]. Also the mechanochemical synthesis, which is currently an inevitable method for the synthesis of chalcogenides [16], was successfully applied for the synthesis of copper sulphides [17-19]. This particular route is in the spotlight of researchers these days, as it offers non-toxic and effective way of preparation. Recently, our group published a paper on the ultrafast mechanochemical synthesis of CuS and Cu2S from elemental precursors, in which the possibility to prepare the desired compounds when milled on air was outlined [20]. In further study, the kinetics of the phase transformations and chemical changes in the products prepared in the explosive manner were further investigated [21]. Copper sulfides are known to possess extraordinary photocatalytic activity [2229]. Regarding the combination of mechanochemistry and mechanochemical synthesis of CuS and evaluation of photocatalytic activity, a report by Li, et al. has to be mentioned [19]. The indirect application of mechanochemistry was applied in [30], where it was used only for the doping of Ce2O3 into ZnS/CuS and subsequently, its photocatalytic activity was pursued. It is known that milling can produce nanoparticles with extraordinary properties [31]. There is a rich plethora of methods for their characterization [32]. These techniques are also interesting for copper sulfides. Mainly those describing microstructure, surface and optical properties or thermal stability can be outlined, as they predetermine the possible application of prepared products in their application field. In the present report, we provide a detailed analysis of the majority of important properties of mechanochemically prepared covellite and chalcocite nanocrystals and provide a proof of their high photocatalytic activity. The photocatalytic activity of the mechanochemically prepared Cu2S nanocrystals was not reported until now and moreover, the surface properties of both compounds prepared by this technique were also not investigated in detail. 2. Characterization methods

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The experimental details regarding the used characterization methods can be found in the Electronic Supplementary Information (ESI).

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3. Results and discussion

The prepared copper sulfides were synthetized by a mechanochemical approach starting from elemental precursors. The explosive character of the reaction and the products after the explosion were described in [20]. Milling brought about further changes in the phase composition, which was the main topic of a subsequent paper [21]. By means of XRD, it was found that almost pure covellite, CuS and chalcocite, Cu2S, can be synthetized within 15 and 30 min, respectively. However, it is well-known that due to the detection limit of this method (< 5 %), it is impossible to detect minor substances, which could be present at the surface. Therefore, the utilization of more sensitive methods (e.g. XPS, IR, ZP or N2 adsorption) is appropriate and the results are presented in this paper. The microstructure and potential application as optical materials and photocatalysts is also investigated.

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ACCEPTED MANUSCRIPT 3.1 Microstructural characterization 3.1.1 SEM, EDX and elemental mapping

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The morphology of the prepared products was monitored utilizing SEM microscopy. The low-mag SEM images (see ESI- Fig. S1) demonstrate that the powder product is composed of large agglomerated grains of size of micro-meter range. This is supported also by the results of grain size analysis reported in [21]. However, these grains contain nanocrystals, as determined by Rietveld analysis from XRD in the same paper and as will also be shown later in this article. According to more magnified SEM images (Fig. 1), it can be seen that the surface of such agglomerate in the case of CuS is quite rich on lamellae and contains many small crystallites. These could possibly represent individual nanoparticles. In the case of Cu2S, the surface is quite smooth and individual nanoparticles are embedded into this smooth matrix. The EDX spectra (presented as insets in Fig. 1) confirm the presence of only copper and sulfur in both samples, the Cu:S ratio being much higher in the case of Cu2S sample (inset in Fig. 1b), as anticipated. The elemental mapping (see ESI- Fig. S2) shows the homogeneous distribution of both copper and sulfur in the structure of both products, meaning that the products do not contain non-reacted precursors.

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Fig. 1: SEM images and EDX spectra of mechanochemically prepared (a) CuS and (b) Cu2S. 3.1.2 Particle size distribution Particle size distribution was measured using a photon cross correlation spectroscopy. The main fraction of both the samples lies in the range from 300 nm to 1200 nm. This fraction is composed of four sub-fractions. In a case of Cu2S sample, the 4

ACCEPTED MANUSCRIPT distribution also contains a fraction of large agglomerated particles, which were not segregated, neither after sonication. The distributions are shown in the supplementary material (Fig. S3) and correlate quite well with the SEM photographs (Fig. 1). The average hydrodynamic particle diameter values, x50, were calculated for both samples and it was found that they were 680 nm and 617 nm for Cu2S and CuS, respectively, thus confirming the finer character of CuS.

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3.2 Optical properties 3.2.1 UV-Vis and PL spectra

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The optical properties of the mechanochemically synthesized products were recorded by means of UV-Vis absorption and photoluminescence (PL) spectroscopy (Fig. 2).

Fig. 2: (a) UV-Vis absorption spectra (inset Tauc’s plots) and (b) PL emission spectra of mechanochemically synthesized CuS and Cu2S

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Fig. 2a shows the UV–Vis absorption spectra of the mechanochemically synthesized CuS and Cu2S dispersed in ethanol. The optical band gaps were determined by plotting (αhν)2 against (hν) and extrapolating the slope in the band edge region to zero, as shown in the inset of Fig. 2a. The optical bandgap of CuS (blue) was evaluated to be 1.92 eV. It is in accordance with the papers [33-35]. Compared with bulk covellite CuS, which has a characteristic absorption band in the near-IR region [36, 37], we observed a blue shift in the band gap energy. The blue shift might be due to the quantum confinement effect exerted by the very small crystallites of the mechanochemically synthesized CuS. In the case of Cu2S (inset of Fig. 2a, red), the calculated optical bandgap was about 3 eV for the direct transition. This is in accordance with the values reported for Cu2S in literature, concretely 2.6 eV [38], 2.7 eV [39] and 3.5 eV [40]. PL spectra of the mechanochemically synthesized CuS and Cu2S excited by 350 nm are shown in Fig. 2b. CuS sample (blue line) has a slight emission at about 400 nm (3.08 eV), which is almost consistent with the literature [41]. The observed emission is in agreement with the photoluminescence observed in paper [42], which results from electron-hole recombination in the surface states. On the other hand, our results are different from previous papers, where no emission [43, 44] and emission in the visible region [45, 46] was reported. The size and morphology of the different CuS products may be responsible for the absence or shift of peaks in spectra. The mechanochemically synthesized Cu2S (Fig. 2b, red line) has a strong green emission band centered at 550 nm (2.24 eV) which is in accordance with that of Cu2S nanoparticles reported in the literature [47]. It is blue-shifted in comparison with the bulk Cu2S which has a bandgap 1.2 eV [48]. It can be attributed to the smaller size of particles. The green emission band may be due to radiative recombination between the conduction band and the wide spread copper-vacancy-related acceptor levels around the valence band edge. It can be seen that the prepared products have the different optical properties. It is in a good accordance with the results reviewed in the paper [49]. It follows from that the bandgap energy depends on the crystalline structure, size as well as morphology of the copper sulfide nanostructures. 3.2.2 Infrared spectra The infrared spectra of the prepared CuS and Cu2S were also measured (see ESIFig. S4). The most important peak describing most probably the Cu-S vibration in CuS is located at 621 cm-1, whereas in Cu2S sample, it was not detected. The position of this peak is in accordance with [50], although the authors reported this peak for Cu1.1Fe1.1.S2 crystals. In their case, the peak for CuS was located at 694 cm-1, but it was quite broad and from our point of view, the sharp peak at 624 cm-1 could be present also for CuS nanocrystals in their case. It was most probably overlapped by some other broad peak. 3.3 Surface characterization 3.3.1 Adsorption/desorption isotherms and pore size distribution 6

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In order to investigate the surface properties of the prepared nanoparticles, the N2 adsorption/desorption isotherms and pore size distribution were measured and the results are presented in the ESI (Fig. S5). The specific surface area values already reported in [21] were 2.7 m2.g-1 and 1.4 m2.g-1 for CuS and Cu2S, respectively, which suggest quite poor surface properties. They should be slightly better in the case of CuS. This is further supported by our new results, as more nitrogen was adsorbed by this sample. As there is no hysteresis loop between the adsorption and desorption curve (Fig. S5a), mesopores should be present in neither of the samples [51]. The shape of the isotherm in the area of relative pressure around 1 suggests only the presence of macropores in the case of CuS sample. Cu2S seems almost completely non-porous. The pore size distribution results (Fig. S5b) further confirm this observation, as the maximum for pore radius for CuS sample is registered around 45 nm, meaning that macropores are present. In the case of Cu2S sample, the y value is almost zero, meaning that almost no pores are present in this sample. This is in accordance with the smooth surface of this sample observed by SEM (Fig. 1b).

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3.3.2 XPS A very effective tool to distinguish between different CuxSy species is the XPS. The Cu 2p and S 2p XPS spectra of the prepared products are compared in Fig. 3a and Fig. 3b, respectively.

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Fig. 3: XPS spectra of CuS and Cu2S: (a) Cu 2p; (b) S 2p.

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In the Cu 2p spectrum (Fig. 3a), a clear difference between the two samples can be seen. The detected signals for Cu in the spectrum for CuS are in accordance with literature, concretely the peaks at 952.0 eV and 932.0 eV are characteristic for the nanocrystals of this compound [19, 52]. Moreover, a satellite peak located at 944.4 eV provides another proof for the presence of copper in oxidation state (II) [52, 53]. Nevertheless, also an example in the literature can be mentioned, in which the oxidation state of Cu in covellite is reported as (I) [54], and as the peaks corresponding to these two forms are quite close, they cannot be completely distinguished in the present case. In addition, our CuS product contains also digenite impurity [21], which could also contribute to the resulting spectra. The spectrum of the Cu2S sample does not contain the signals which are anticipated, as these should be located at slightly higher binding energies than in the case of CuS [55]. The observed peaks could hint to the oxidation of Cu2S to CuSO4 and copper oxides at the surface, which is a common phenomenon [56]. As the XPS measurements were not performed under ion gun, the surface impurities can 8

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significantly affect the result. The peak at 956.1 eV is quite unexpected and was reported in literature when Cu was engaged in complex compounds [57, 58]. The peak at 944.1 eV could be present because of CuO formation on the surface [59]. The peak at 936.1 eV could document the presence of CuSO4 [60]. The S 2p spectra (Fig. 3b) confirm the facts observed from the Cu 2p spectra. The main peak located at 162.8 eV in the case of CuS is a proof of the successful CuS synthesis [19, 52]. The peaks located at higher binding energies could mean slight oxidation (CuSO4 exhibits peaks in this region) [53], but these were observed also in the paper [52]. In the case of Cu2S sample, the peaks at 161.3 eV and 163.3 eV could be associated with sulfur bound in the form of sulfides [53]. The peak located at 166.5 eV is peculiar, as possibly elemental sulfur [61], or some sulfinate species [62] could be present. Nevertheless, it is a proof of the decomposition of the Cu2S surface. The obtained results are in good correlation with the zeta potential measurements described below (Fig. 4).

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3.3.3. Zeta potential The zeta potential in dependence of applied pH (in a range from 3 to 12) was measured for both studied samples. The results are depicted in Fig. 4.

Fig. 4: Zeta potential vs. pH of mechanochemically synthesized CuS and Cu2S In the case of CuS sample, all the zeta potential values lie in negative area and the isoelectric point was not detected in a measured pH range. According to literature, most unoxidised or just lightly oxidised metal sulfides have isoelectric point values below 3.0 [63], concretely, the values below 4 [64], 2 [54, 65], or 1 [63] were reported. Our measurements are in accordance with these results, as the isoelectric point was detected at the pH value lower than 3.0 The shape of the zeta potential vs. pH curve is very similar to that reported in [65]. The values of zeta potential for CuS become dependent on the density and partial charge of functional groups. The presence of unoxidised metal-deficient sulfides or polysulfides that show dangling bonds [66], on the CuS sample surface is consistent with the negative sign of the zeta potential. 9

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The zeta potential vs. pH curve for Cu2S sample is shifted to the region with less negative values in comparison to CuS one. It is connected with higher amount of copper ions in a structure and deficiency of sulfur ions in comparison to CuS sample, leading to the absence of metal-deficient sulfides or polysulfides, which results in less negative charge. On the other hand, it could be a result of surface oxidation. Upon oxidation, the sulfide mineral surface becomes increasingly covered with metal oxide/hydroxide species and the zeta potential versus pH curves of such sulfide mineral become less negative, or even positive [67, 68]. Moreover, from the detailed analysis of the effects of pH, or oxidizing conditions on the zeta potential of copper sulfide minerals, it was shown that the oxidation of chalcocite, Cu2S is easier in comparison to oxidation of covellite, CuS [64], which could be the reason of identifying the isoelectric point at pH 3.17 for Cu2S sample. This is in accordance with the XPS and FTIR measurements.

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3.4 Thermal stability To investigate the thermal stability of the mechanochemically synthesized CuS and Cu2S, the thermal analysis in the absence of oxidative conditions was performed. Fig. S6 in the ESI shows the TG/DTG-DTA curves for both mechanochemically synthesized materials. As can be seen, in the case of CuS (Fig. S6a), a two-step decomposition process welldefined with DTA endotherm peaks occurred in the temperature range of 210-310 °C and 400-515 °C with 0.9% and 13.4% weight loss. Both steps can be related to the release of volatile sulfur to liquid polymeric phase. This idea is also supported by XRD analysis. In the XRD patterns (see ESI Fig. S7) of the thermally treated CuS, the presence of digenite, Cu1.8S (JCPDS PDF 23-0962) and chalcocite, Cu2S (JCPDS PDF 01-072-1071) phases is evidenced. On the other hand, mechanochemically synthesized Cu2S is thermally stable up to 330 °C (Fig. S6b). Above this temperature, a one-step decomposition process occurred with 1.3 % weight loss. The XRD analysis of the sample after thermal treatment revealed the presence of Cu2S indicating no change in stoichiometry. A small mass loss can be associated with the decomposition of small amount of various species present on the surface of this sample, which is in accordance with the XPS and ZP measurements.

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ACCEPTED MANUSCRIPT 3.5 Photocatalytic activity

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Photocatalytic activity of mechanochemically synthesized copper sulfides was conducted using Methyl Orange (MO) as a model. The dark decolorization test of MO solution was first carried out to check the adsorption ability of the prepared materials. The adsorption-desorption equilibrium of MO dye molecules was achieved after stirring for 30 min. The sample Cu2S (0.133 mg.g-1) possessed better adsorption capacity than CuS (0.034 mg.g-1) sample. Fig. 5 shows the photocatalytic degradation efficiency of the dye as function of time.

Fig. 5: The irradiation time-dependent degradation efficiencies of MO solution in the presence of CuS and Cu2S photocatalysts.

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Cu2S was found to be more effective photocatalyst, as complete degradation of MO dye was observed within 150 min. CuS showed 80 % MO degradation in the same reaction time, respectively. For comparison, the photocatalytic tests were performed also with the commercial TiO2 Degussa P25 powder and commercial ZnO under similar degradation conditions. The results, together with the ones for commercial ZnO reported in two other studies, are compared with those of mechanochemically synthesized copper sulfides in Table 1. Commercial ZnO was used for comparison as it is one of the most common photocatalysts [69-71]. Table 1 Comparison of conversion of MO after 150 min visible light irradiation Sample CuS Cu2S Commercial TiO2 P25 Commercial ZnO (Bulgaria) Commercial ZnO (China) [72]

Conversion (%) 0.80 0.98 0.04 0.05 0.07 11

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The photocatalytic activity of degradation of Methyl Orange under visible light irradiation for the commercial photocatalysts was observed to be much lower and could degrade only 4 % and 5 % MO solution within the same irradiation time for P25 and ZnO, respectively. Also the photocatalytic activities of commercial ZnO reported in other studies were much lower [72, 73]. This demonstrates that the photocatalytic activity is obviously dependent on the structures of the samples. The linear relationships of ln(Co/C) versus irradiation time reveals that the photodegradation reaction follows a pseudo-first-order reaction. The calculated reaction rate constant for Cu2S is 0.015 min-1 and that of CuS is 0.0135 min-1. The measured values of the specific surface area reported in [21] of the prepared products are close, which also explains quite similar photocatalytic activities. Probably, the lower photocatalytic activity of CuS is due to the formation of small quantity of sulfate species on the surface of this sample in air atmosphere during photocatalytic test. As this sample is finer, more species causing deterioration of the photocatalytic activity can be formed. Moreover, the presence of oxidized species in the case of Cu2S sample (as was shown mainly be the XPS results) could facilitate the decolorization of dye. The proposed mechanism of converting solar energy to chemical energy in photocatalysis of CuxS (x=1 or 2) is similar with that of extensively researched TiO2 photocatalyst. The electrons and holes could be excited under visible irradiation to the conduction band and the valence band edge, respectively. These photo-excited slectrons and holes can be transferred to the surface of the CuxS photocatalyst, where they react with oxidant (O2) and reductant (OH-), respectively. For example, the excited electron reacts with surface dissolved O2 to form
О2- super oxide anion-radical and hole reacts with surface hydroxyl groups to form hydroxyl radicals (
OH) [73]. The formed
О2further reacts with adsorbed H2O to produce H2O2, which provides hydroxyl radical (•OH) by acting as a direct electron acceptor by reaction with electron and
О2- . Additional •OH radicals are formed by the chemical reaction between Cu2S and H2O2 (eq. 1). Cu+ + H2O2 → Cu2+ + OH- + •OH

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(1)

This reaction does not take place in the case of CuS. Therefore, more hydroxyl radicals are formed in presence of Cu2S than CuS, which explains the higher photocatalytic activity of the former sample (Fig. 5). These hydroxyl radicals (•OH) exhibit extremely strong oxidizing properties for partial or complete mineralization of organic chemicals and are able to degrade the MO pollutant [74]. Organic molecules of MO could be decomposed through an oxidation reaction by the formed
О2- or
OH or directly by the holes accumulated in the valence band.

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ACCEPTED MANUSCRIPT Conclusion

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The detailed characterization of the mechanochemically synthesized covellite CuS and chalcocite Cu2S was performed. The morphology study using SEM revealed the presence of agglomerates in micrometer range containing nanoparticles. The EDS analysis and elemental mapping has provided information about homogeneous distribution of both elements. The particle size distribution correlates very well with SEM micrographs. The optical properties have shown that the prepared materials could be suitable for optoelectronic applications due to acceptable band-gap energies (1.92 eV for CuS and 3 eV for Cu2S). The infrared spectra of CuS revealed the presence of characteristic peak at 621 cm-1. The surface properties analysis has shown that the CuS sample is mostly macroporous and Cu2S is non-porous. The XPS measurements have confirmed the structure of CuS nanocrystals and the surface oxidation of Cu2S. This surface oxidation was also confirmed by zeta potential measurements. The thermal stability of CuS is lower, as it is decomposed into chalcocite and digenite, whereas Cu2S is not decomposed. The photocatalytic activity of both CuS and Cu2S was much higher than in the case of commercial TiO2 and ZnO. Regarding the comparison of sulfides, Cu2S was slightly better photocatalyst. Acknowledgements

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References

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This work was supported by the Slovak Research and Development Agency under the contracts No. APVV14-0103 and SK-BG-2013-0011. The support of Slovak Grant Agency VEGA (projects 2/0044/18 and 2/0065/18) and of the European Regional Development Fund (project ITMS 26220120035) is also gratefully acknowledged. The authors gratefully acknowledge the financial support by the National Science Fund (Bulgaria) under the Contract DNTS/Slovakia 01/2. The authors express their gratitude to Dr. Anna Zorkovská for XRD measurements.

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[1] S. Goel, F. Chen, W.B. Cai, Synthesis and biomedical applications of copper sulfide nanoparticles: From sensors to theranostics, Small, 10 (2014) 631-645. [2] Y.B. Li, W. Lu, Q.A. Huang, M.A. Huang, C. Li, W. Chen, Copper sulfide nanoparticles for photothermal ablation of tumor cells, Nanomedicine, 5 (2010) 1161-1171. [3] Z.K. Yang, L.X. Song, Y. Teng, J. Xia, Ethylenediamine-modulated synthesis of highly monodisperse copper sulfide microflowers with excellent photocatalytic performance, J. Mater. Chem. A, 2 (2014) 20004-20009. [4] S. Farhadi, F. Siadatnasab, Copper(I) sulfide (Cu2S) nanoparticles from Cu(II) diethyldithiocarbamate: Synthesis, characterization and its application in ultrasound-assisted catalytic degradation of organic dye pollutants, Mater. Res. Bull., 83 (2016) 345-353. [5] F.A. Sabah, N.M. Ahmed, Z. Hassan, H.S. Rasheed, High performance CuS p-type thin film as a hydrogen gas sensor, Sensor. Actuat. A-Phys., 249 (2016) 68-76. [6] A. Cuevas, R. Romero, E.A. Dalchiele, J.R. Ramos-Barrado, F. Martin, D. Leinen, Spectrally selective CuS solar absorber coatings on stainless steel and aluminum, Surf. Interface Anal., 48 (2016) 649-653. [7] C.F. Pan, S.M. Niu, Y. Ding, L. Dong, R.M. Yu, Y. Liu, G. Zhu, Z.L. Wang, Enhanced Cu2S/CdS coaxial nanowire solar cells by piezo-phototronic effect, Nano Lett., 12 (2012) 3302-3307. [8] Y. Wu, C. Wadia, W.L. Ma, B. Sadtler, A.P. Alivisatos, Synthesis and photovoltaic application of copper(I) sulfide nanocrystals, Nano Lett., 8 (2008) 2551-2555. [9] R. Mulla, M.K. Rabinal, Ambient growth of highly oriented Cu2S dendrites of superior thermoelectric behaviour, Appl. Surf. Sci., 397 (2017) 70-76. [10] L.J. Zheng, B.P. Zhang, H.Z. Li, J.B. Yu, CuxS superionic compounds: Electronic structure and thermoelectric performance enhancement, J. Alloys Compd., 722 (2017) 17-24.

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[11] Y. Xie, L. Carbone, C. Nobile, V. Grillo, S. D'Agostino, F. Della Sala, C. Giannini, D. Altamura, C. Oelsner, C. Kryschi, P.D. Cozzoli, Metallic-like stoichiometric copper sulfide nanocrystals: Phase- and shapeselective synthesis, near-infrared surface plasmon resonance properties, and their modeling, ACS Nano, 7 (2013) 7352-7369. [12] P. Roy, S.K. Srivastava, Hydrothermal growth of CuS nanowires from Cu-dithiooxamide, a novel singlesource precursor, Cryst. Growth Des., 6 (2006) 1921-1926. [13] Y.Q. Zhang, B.P. Zhang, Z.H. Ge, L.F. Zhu, Y. Li, Preparation by solvothermal synthesis, growth mechanism, and photocatalytic performance of CuS nanopowders, Eur. J. Inorg. Chem., 2014 (2014) 2368-2375. [14] A. Ghahremaninezhad, E. Asselin, D.G. Dixon, One-step template-free electrosynthesis of 300 µm long copper sulfide nanowires, Electrochem. Commun., 13 (2011) 12-15. [15] J. Liu, D. Xue, Rapid and scalable route to CuS biosensors: a microwave-assisted Cu-complex transformation into CuS nanotubes for ultrasensitive nonenzymatic glucose sensor, J. Mater. Chem., 21 (2011) 223-228. [16] P. Baláž, M. Baláž, M. Achimovičová, Z. Bujňáková, E. Dutková, Chalcogenide mechanochemistry in materials science: insight into synthesis and applications (a review), J. Mater. Sci., 52 (2017) 11851-11890. [17] J.V. Tolia, M. Chakraborty, Z.V.P. Murthy, Mechanochemical synthesis and characterization of group II-VI semiconductor nanoparticles, Particul. Sci. Technol., 30 (2012) 533-542. [18] M. Kristl, I. Ban, S. Gyergyek, Preparation of nanosized copper and cadmium chalcogenides by mechanochemical synthesis, Mater. Manuf. Processes, 28 (2013) 1009-1013. [19] S. Li, Z.H. Ge, B.P. Zhang, Y. Yao, H.C. Wang, J. Yang, Y. Li, C. Gao, Y.H. Lin, Mechanochemically synthesized sub-5 nm sized CuS quantum dots with high visible-light-driven photocatalytic activity, Appl. Surf. Sci., 384 (2016) 272-278. [20] M. Baláž, A. Zorkovská, F. Urakaev, P. Baláž, J. Briančin, Z. Bujňáková, M. Achimovičová, E. Gock, Ultrafast mechanochemical synthesis of copper sulfides, RSC Adv., 6 (2016) 87836-87842. [21] M. Baláž, A. Zorkovská, J.S. Blazquez, N. Daneu, P. Baláž, Mechanochemistry of copper sulphides: Phase interchanges during milling, J. Mater. Sci., 52 (2017) 11947-11961. [22] Z.Y. Zhang, J. Zhang, T. Wu, X.H. Bu, P.Y. Feng, Three-dimensional open framework built from Cu-S icosahedral clusters and its photocatalytic property, JACS, 130 (2008) 15238-+. [23] M. Basu, A.K. Sinha, M. Pradhan, S. Sarkar, Y. Negishi, Govind, T. Pal, Evolution of hierarchical hexagonal stacked plates of CuS from liquid-liquid interface and its photocatalytic application for oxidative degradation of different dyes under indoor lighting, Environ. Sci. Technol., 44 (2010) 6313-6318. [24] M. Saranya, C. Santhosh, R. Ramachandran, P. Kollu, P. Saravanan, M. Vinoba, S.K. Jeong, A.N. Grace, Hydrothermal growth of CuS nanostructures and its photocatalytic properties, Powder Technol., 252 (2014) 2532. [25] X.S. Hu, Y. Shen, L.H. Xu, L.M. Wang, L.S. Lu, Y.T. Zhang, Preparation of flower-like CuS by solvothermal method for photocatalytic, UV protection and EMI shielding applications, Appl. Surf. Sci., 385 (2016) 162-170. [26] X.S. Hu, Y. Shen, L.H. Xu, L.M. Wang, Y.J. Xing, Preparation of flower-like CuS by solvothermal method and its photodegradation and UV protection, J. Alloys Compd., 674 (2016) 289-294. [27] N. Sreelekha, K. Subramanyam, D.A. Reddy, G. Murali, S. Ramu, K.R. Varma, R.P. Vijayalakshmi, Structural, optical, magnetic and photocatalytic properties of Co doped CuS diluted magnetic semiconductor nanoparticles, Appl. Surf. Sci., 378 (2016) 330-340. [28] C.D. Song, X. Wang, J. Zhang, X.B. Chen, C. Li, Enhanced performance of direct Z-scheme CuS-WO3 system towards photocatalytic decomposition of organic pollutants under visible light, Appl. Surf. Sci., 425 (2017) 788-795. [29] X.S. Hu, Y. Shen, Y.T. Zhang, H.F. Zhang, L.H. Xu, Y.J. Xing, Synthesis of flower-like CuS/reduced graphene oxide (RGO) composites with significantly enhanced photocatalytic performance, J. Alloys Compd., 695 (2017) 1778-1785. [30] C. Shifu, J. Mingsong, Y. Yunguang, Synthesis and characterization of Ce2S3-ZnS-CuS nanoparticles and their photocatalytic activity, J. Nanosci. Nanotechnol., 12 (2012) 4898-4904. [31] P. Baláž, M. Achimovičová, M. Baláž, P. Billik, Z. Cherkezova-Zheleva, J.M. Criado, F. Delogu, E. Dutková, E. Gaffet, F.J. Gotor, R. Kumar, I. Mitov, T. Rojac, M. Senna, A. Streletskii, K. Wieczorek-Ciurowa, Hallmarks of mechanochemistry: from nanoparticles to technology, Chem. Soc. Rev., 42 (2013) 7571-7637. [32] G. Schmid, Nanoparticles: From Theory to Application, 2nd ed., Wiley-VCH, Hoboken, USA, 2010. [33] J. Zhang, Z. Zhang, Hydrothermal synthesis and optical properties of CuS nanoplates, Mater. Lett., 62 (2008) 2279-2281. [34] L.F. Chen, W. Yu, Y. Li, Synthesis and characterization of tubular CuS with flower-like wall from a low temperature hydrothermal route, Powder Technol., 191 (2009) 52-54. [35] X.P. Shen, H. Zhao, H.Q. Shu, H. Zhou, A.H. Yuan, Self-assembly of CuS nanoflakes into flower-like microspheres: Synthesis and characterization, J. Phys. Chem. Solids, 70 (2009) 422-427.

14

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[36] S.K. Haram, A.R. Mahadeshwar, S.G. Dixit, Synthesis and characterization of copper sulfide nanoparticles in Triton-X 100 water-in-oil microemulsions, J. Phys. Chem., 100 (1996) 5868-5873. [37] S.G. Dixit, A.R. Mahadeshwar, S.K. Haram, Some aspects of the role of surfactants in the formation of nanoparticles, Colloids Surf., A, 133 (1998) 69-75. [38] A. Vasuhi, R.J. Xavier, R. Chandramohan, S. Muthukumaran, K. Dhanabalan, M. Ashokkumar, P. Parameswaran, Effect of heat treatment on the structural and optical properties of Cu2S thin films deposited by CBD method, J. Mater. Sci. - Mater. Electron., 25 (2014) 824-831. [39] S. Hemathangam, G. Thanapathy, S. Muthukumaran, Tuning of band gap and photoluminescence properties of Zn doped Cu2S thin films by CBD method, J. Mater. Sci. - Mater. Electron., 27 (2016) 2042-2048. [40] D.K. Li, J.J. Ma, L.F. Zhou, Y. Li, C.W. Zou, Synthesis and characterization of Cu2S nanoparticles by diethylenetriamine-assisted hydrothermal method, Optik, 126 (2015) 4971-4973. [41] S.M. Ou, Q. Xie, D.K. Ma, J.B. Liang, X.K. Hu, W.C. Yu, Y.T. Qian, A precursor decomposition route to polycrystalline CuS nanorods, Mater. Chem. Phys., 94 (2005) 460-466. [42] C. Nascu, I. Pop, V. Ionescu, E. Indrea, I. Bratu, Spray pyrolysis deposition of CuS thin films, Mater. Lett., 32 (1997) 73-77. [43] X.C. Jiang, Y. Xie, J. Lu, W. He, L.Y. Zhu, Y.T. Qian, Preparation and phase transformation of nanocrystalline copper sulfides (Cu9S8, Cu7S4 and CuS) at low temperature, J. Mater. Chem., 10 (2000) 21932196. [44] A.M. Qin, Y.P. Fang, H.D. Ou, H.Q. Liu, C.Y. Su, Formation of various morphologies of covellite copper sulfide submicron crystals by a hydrothermal method without surfactant, Cryst. Growth Des., 5 (2005) 855-860. [45] F. Li, T. Kong, W.T. Bi, D.C. Li, Z. Li, X.T. Huang, Synthesis and optical properties of CuS nanoplatebased architectures by a solvothermal method, Appl. Surf. Sci., 255 (2009) 6285-6289. [46] F. Li, J.F. Wu, Q.H. Qin, Z. Li, X.T. Huang, Controllable synthesis, optical and photocatalytic properties of CuS nanomaterials with hierarchical structures, Powder Technol., 198 (2010) 267-274. [47] M. Salavati-Niasari, E. Esmaeili, M. Sabet, Synthesis and characterization of Cu2S nanostructures via hydrothermal method by a polymeric precursor, J. Cluster Sci., 24 (2013) 799-809. [48] N. Li, X.L. Zhang, S.T. Chen, W. Yang, H.Z. Kang, W.H. Tan, One-pot self-assembly of flower-like Cu2S structures with near-infrared photoluminescent properties, CrystEngComm, 13 (2011) 6549-6554. [49] P.V. Quintana-Ramirez, M.C. Arenas-Arrocena, J. Santos-Cruz, M. Vega-Gonzalez, O. Martinez-Alvarez, V.M. Castano-Meneses, L.S. Acosta-Torres, J. de la Fuente-Hernandez, Growth evolution and phase transition from chalcocite to digenite in nanocrystalline copper sulfide: Morphological, optical and electrical properties, Beilstein J. Nanotechnol., 5 (2014) 1542-1552. [50] L.Z. Pei, J.F. Wang, X.X. Tao, S.B. Wang, Y.P. Dong, C.G. Fan, Q.F. Zhang, Synthesis of CuS and Cu1.1Fe1.1S2 crystals and their electrochemical properties, Mater. Charact., 62 (2011) 354-359. [51] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting physisorption data for gas solid systems with special reference to the determination of surface area and porosity (Recommendations 1984), Pure Appl. Chem., 57 (1985) 603-619. [52] M.D. Ye, X.R. Wen, N. Zhang, W.X. Guo, X.Y. Liu, C.J. Lin, In situ growth of CuS and Cu1.8S nanosheet arrays as efficient counter electrodes for quantum dot-sensitized solar cells, J. Mater. Chem. A, 3 (2015) 95959600. [53] V. Krylova, M. Andrulevicius, Optical, XPS and XRD studies of semiconducting copper sulfide layers on a polyamide film, Int. J. Photoenergy, 2009 (2009) art. ID 304308. [54] M.K. Nduna, A.E. Lewis, P. Nortier, A model for the zeta potential of copper sulphide, Colloids Surf., A, 441 (2014) 643-652. [55] P. Kar, S. Farsinezhad, X.J. Zhang, K. Shankar, Anodic Cu2S and CuS nanorod and nanowall arrays: preparation, properties and application in CO2 photoreduction, Nanoscale, 6 (2014) 14305-14318. [56] E.C. Todd, D.M. Sherman, Surface oxidation of chalcocite (Cu2S) under aqueous (pH=2-11) and ambient atmospheric conditions: Mineralogy from Cu L- and O K-edge X-ray absorption spectroscopy, Am. Mineral., 88 (2003) 1652-1656. [57] R.A. Molla, M.A. Iqubal, K. Ghosh, Kamaluddin, S.M. Islam, Nitrogen enriched mesoporous organic polymer anchored copper(II) material: an efficient and reusable catalyst for the synthesis of esters and amides from aromatic systems, Dalton Trans., 44 (2015) 6546-6559. [58] A. Narani, R.K. Marella, P. Ramudu, K.S.R. Rao, D.R. Burri, Cu(II) complex heterogenized on SBA-15: a highly efficient and additive-free solid catalyst for the homocoupling of alkynes, RSC Adv., 4 (2014) 3774-3781. [59] F. Parmigiani, G. Pacchioni, F. Illas, P.S. Bagus, Studies of the Cu-O bond in cupric oxide by X-ray photoelectron spectroscopy and ab initio electronic structure models, J. Electron. Spectrosc. Relat. Phenom., 59 (1992) 255-269. [60] B.R. Strohmeier, D.E. Leyden, R.S. Field, D.M. Hercules, Surface spectroscopic characterization of Cu/Al2O3 catalysts, J. Catal., 94 (1985) 514-530.

15

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

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[61] T. Fujimori, A. Morelos-Gomez, Z. Zhu, H. Muramatsu, R. Futamura, K. Urita, M. Terrones, T. Hayashi, M. Endo, S.Y. Hong, Y.C. Choi, D. Tomanek, K. Kaneko, Conducting linear chains of sulphur inside carbon nanotubes, Nature Communications, 4 (2013) art. ID 2162. [62] F. Laffineur, J. Delhalle, S. Guittard, S. Geribaldi, Z. Mekhalif, Mechanically polished copper surfaces modified with n-dodecanethiol and 3-perfluorooctyl-propanethiol, Colloids Surf., A, 198 (2002) 817-827. [63] J.C. Liu, C.P. Huang, Electrokinetic characteristics of some metal sulfide water interfaces, Langmuir, 8 (1992) 1851-1856. [64] D. Fullston, D. Fornasiero, J. Ralston, Zeta potential study of the oxidation of copper sulfide minerals, Colloids Surf., A, 146 (1999) 113-121. [65] T.P. Mokone, R.P. van Hille, A.E. Lewis, Effect of solution chemistry on particle characteristics during metal sulfide precipitation, J. Colloid Interface Sci., 351 (2010) 10-18. [66] R.A.D. Pattrick, J.F.W. Mosselmans, J.M. Charnock, K.E.R. England, G.R. Helz, C.D. Garner, D.J. Vaughan, The structure of amorphous copper sulfide precipitates: An X-ray absorption study, Geochim. Cosmochim. Acta, 61 (1997) 2023-2036. [67] G. Fairthorne, D. Fornasiero, J. Ralston, Effect of oxidation on the collectorless flotation of chalcopyrite, Int. J. Miner. Process., 49 (1997) 31-48. [68] D. Fornasiero, V. Eijt, J. Ralston, An electrokinetic study of pyrite oxidation, Colloids and Surfaces, 62 (1992) 63-73. [69] R. Saravanan, E. Thirumal, V.K. Gupta, V. Narayanan, A. Stephen, The photocatalytic activity of ZnO prepared by simple thermal decomposition method at various temperatures, J. Mol. Liq., 177 (2013) 394-401. [70] R. Saravanan, V.K. Gupta, V. Narayanan, A. Stephen, Comparative study on photocatalytic activity of ZnO prepared by different methods, J. Mol. Liq., 181 (2013) 133-141. [71] K.Z. Qi, B. Cheng, J.G. Yu, W.K. Ho, Review on the improvement of the photocatalytic and antibacterial activities of ZnO, J. Alloys Compd., 727 (2017) 792-820. [72] C.L. Wu, Y.C. Zhang, Q.L. Huang, Solvothermal synthesis of N-doped ZnO microcrystals from commercial ZnO powder with visible light-driven photocatalytic activity, Mater. Lett., 119 (2014) 104-106. [73] A.B. Lavand, Y.S. Malghe, Synthesis, characterization and visible light photocatalytic activity of nitrogendoped zinc oxide nanospheres, J. Asian Ceram. Soc., 3 (2015) 305-310. [74] M. Stylidi, D.I. Kondarides, X.E. Verykios, Pathways of solar light-induced photocatalytic degradation of azo dyes in aqueous TiO2 suspensions, Applied Catalysis B-Environmental, 40 (2003) 271-286.

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Mechanochemically synthesized CuS and Cu2S were characterized in detail. Both compounds have shown great photocatalytic properties. Both compounds might be suitable for photovoltaic applications. Surface oxidation of Cu2S was observed.

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