Uncovering the mechanism for the formation of copper

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Uncovering the mechanism for the formation of copper thioantimonides (SbIII) and thioantimonates (SbV) nanoparticles Fábio Baum, Tatiane Pretto, Alexandre G. Brolo, and Marcos Jose Leite Santos Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00667 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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Crystal Growth & Design

Uncovering the mechanism for the formation of copper thioantimonate (SbV) nanoparticles and its transition to thioantimonide (SbIII) AUTHOR NAMES Fábio Baum,a Tatiane Pretto,b Alexandre G. Brolo,c* Marcos José Leite Santosa,b* AUTHOR ADDRESS a Programa de Pós-Graduação em Ciências de Materiais, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500. Bairro Agronomia - Porto Alegre/RS - 91501 – 970 - Brazil. E-mail: [email protected] b Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500. Bairro Agronomia - Porto Alegre/RS - 91501 – 970 - Brazil. c Department of Chemistry and Center for Advanced Materials and Related Technologies (CAMTEC), University of Victoria, P.O. Box 3065, V8W 3V6, BC, Canada. E-mail: [email protected]

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ABSTRACT

Ternary sulfide nanostructures are small band gap materials that combine relative low toxicity with useful optical properties for several applications, including photovoltaics. A systematic experimental study on the synthesis and mechanism of formation of copper thioantimonates (Cu3SbS4) and thioantimonides (CuSbS2) nanoparticles is presented. Antimony oxide (Sb2O3) was formed in an initial step by hydrolysis with oleylamine. The injection of a sulfur precursor led to the conversion to Cu3SbS4 driven by an excess of sulfur in oleylamine medium and at high temperatures (> 200 °C). The sulfur excess was depleted as the reaction progressed, causing the reduction of the antimony (V) of the Cu3SbS4 back to antimony (III). Consequently, the Cu3SbS4 was converted to CuSbS2. In addition, the rate of antimony reduction increased with the reaction temperature. The formation mechanism unveiled here provides important insights towards the synthesis of analogous materials.


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INTRODUCTION Semiconductor nanocrystals (SC-NC) have been widely studied due to their potential for applications in light-emitting diodes,[1-3] biological labelling,[4,5] sensors,[6-8] photocatalysts,[9,10] and solar cells.[11,12] However, most of the SC-NCs present toxic heavy metals in their composition, such as cadmium and lead. Although toxic materials are required to yield the desirable physical properties, environmental and health risks are major concerns in this field. Recently, growing attention is being given to low toxic SC-NCs, such as CuInS2,[13] CuInSe2,[14] and AgInS2.[15,16] Nevertheless, indium is a rare and expensive element, which is certainly a major drawback towards large-scale applications. To overcome this challenge, Indium has been substituted by cheaper and more abundant elements, such as tin and antimony.[17-19] Within this context, ternary and quaternary metal sulfides have emerged as an economically viable option to tackle the sustainability and toxicity problem of quantum dots.[20,21] The ternary sulfides encompass a large class of materials, with over 200 different compounds in nature, and their bandgap mostly lies between 0.8 and 2.0 eV.[12,22] The wide range of bandgaps, combined with a large availability, make these compounds an important class of material. Recently, growing attention is been dedicated to thioantimony compounds, including CuSbS2 (chalcostibite), Cu3SbS3 (skinnerite and wittichenite), Cu12Sb4S13 (tetrahedrite), and Cu3SbS4 (famatinite).[23-27] Control of the synthesis of thioantimony compounds

could warrant the

production of low toxicity SC-NCs with high chemical stability and compositional-tunable optical band gaps. In this work, a systematic investigation of the synthesis and formation mechanism of thioantimony-based SC-NCs, synthesized by the hot injection method, was carried out.


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Intermediate compounds formed during the synthesis were identified, as well as the role of the temperature into the final composition of the material, including the oxidation state of antimony. The results presented here allow the prediction of the synthesis conditions and the control of the formation of copper thioantimonate and thioantimonide nanoparticles and similar materials. EXPERIMENTAL Materials Copper (I) chloride

(Sigma-Aldrich, 97%), antimony (III) chloride (Sigma-Aldrich, 99%),

Oleylamine (Sigma-Aldrich), Oleic Acid (Sigma-Aldrich), Sulphur (Sigma-Aldrich), hexane (Synth) and ethanol (Synth) were used as received. Synthesis of CuxSbSy nanoparticles. 0.45 mmol of copper (I) chloride, 0.45 mmol of antimony (III) chloride and 7 mL of oleylamine were added to a three-neck glass flask and degassed at 80 °C for 30 minutes. The solution was then heated up to the reaction temperature (200 – 250 °C) under argon atmosphere. In another flask, 1 mmol sulphur and 3 mL of olyelamine were heated to 60 °C, degassed and stirred under argon atmosphere for nearly 30 minutes, until the solution presented reddish transparent color. After temperature stabilization, 3 mL of the sulphur solution was injected into the three-neck flask. Aliquots were taken after 1, 2, and 5 minutes and the reaction was stopped by removing the heat source and cooling down to the room temperature after 10 minutes. Oleic acid and hexane were added, and the samples were centrifuged for 10 minutes at 2500 RPM. Following, the samples were rinsed with ethanol and centrifuged for 10 minutes at 2500 RPM. The


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supernatant was discarded and the nanocrystals were dispersed with hexane for further analysis. All of the procedures were performed in a standard Schlenk line. Characterization Transmission electron microscopy (TEM) images were obtained in a Jeol JEM 1200 EXll with the acceleration voltage of 80 kV. The selected area electron diffraction (SAED) and highresolution transmission electron microscopy (HRTEM) images were obtained in a JEOL JEM 2010 with the acceleration voltage of 200 kV. The samples were deposited by drop casting a dispersion of the nanoparticles on a carbon coated copper grid of 200 mesh. UV-Vis-NIR spectroscopy was carried out in a Cary 5000 UV-Vis-NIR spectrometer using a standard 10 mm quartz cuvette. X-ray diffraction patterns were obtained in a Siemens diffractometer with a Kα radiation (λ= 1.54 Å) from 10 to 80 degrees, with an increment 0.05°. Raman spectroscopy was performed in a Renishaw InVia Raman confocal microscope, equipped with a 50x microscope objective, and a 633 nm laser. Thin films of the samples were deposited by drop casting onto microscope slide glasses for the Raman measurements. The acquisition conditions were 20 scans of 20 seconds each. RESULTS AND DISCUSSION The Raman spectra in Figure 1 shows that either Cu3SbS4 and/or CuSbS2 phases are formed under all reaction conditions. The Raman peak centered at ~319 cm-1 is characteristic of the famatinite (Cu3SbS4) phase, while the vibration band at ~336 cm-1 is characteristic to chalcostibite (CuSbS2). Figure 1 shows that the Cu3SbS4 phase dominates at early stages of the reaction (after 1 minute) for all temperatures. On the other hand, the CuSbS2 phase predominates after 10 min of reaction. In general, Figure 1 shows a trend for the formation of Cu3SbS4 at


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shorter reaction times (1 and 2 minutes), while CuSbS2 is formed after longer reaction periods (5 and 10 minutes). It is also possible to observe that at higher temperatures (Figure 1e and 1f) the transition between famatinite and chalcostibite occurs before 2 minutes of reaction. This is much faster than observed for synthesis at lower temperatures, where this transition occurs only after either 10 (Figure 1a) or 5 minutes (Figure 1b).

Figure 1. Raman spectroscopy of the samples synthesized 200 (a), 210 (b), 220 (c), 230 (d), 240 (e), and 250 °C (f). Figure 2 presents the Vis-NIR absorption spectra of the samples synthesized at different temperatures (from 200 to 250 °C), and with reaction times ranging from 1 to 10 minutes. The samples obtained at 200 °C (Figure 2a) present a broad absorption peak with maximum at ca. 1300 nm for 1 and 2 minutes of reaction time. A similar peak at 1500 nm is also seen in Figure 2b (210 °C) after 1 minute of reaction, while this peak shifts to 1600 nm after 2 minutes. At 220 °C, Figure 2c, this broad absorption peak is present at 1500 nm for the sample synthesized at


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1 minute of reaction. All those big broad NIR peaks may be associated with localized surface plasmon resonances (LSPRs) of the Cu3SbS4 phase. Similar LSPR bands have been observed in other copper sulfides[28,29] and interpreted as originating from excess free carriers generated by copper vacancies in the lattice.[16] The presence of the vacancies can be related to the temperature of the synthesis, once the LSPR is not observed in Figure 2 for the Cu3SbS4 phase with temperature of synthesis higher than 230 °C. The results from Figure 2 were used to calculate the optical bandgap, using the Tauc plot (Figure S1 to S12), of all SC-NCs synthesized in this work. The results are summarized in Table S1 on the Support Information. The optical bandgaps for the SC-NCs were between 0.77 to 2.52 eV, which is within the expected range for copper thioantimonate and thioantimonide nanoparticles. The Table S1 in Support Information shows the values and nature of the bandgap of the synthesized SC-NCs. The band gaps were organized by the composition extracted from the Raman spectra. It is possible to observe that the Cu3SbS4 nanoparticles generally present its lower transition as an indirect bandgap, while the bandgap values corroborates the literature, except for the ~2.5 eV direct transition found for both Cu3SbS4 and CuSbS2. The results reveal clear preference for Cu3SbS4 to present the lower transition as an indirect bandgap, and CuSbS2 to present direct bandgap. In contrast, examples of both bandgap characteristics, indirect and direct, have been reported for Cu3SbS4 and CuSbS2.[23-26,30-38,49-55] This suggests that the method of synthesis affects the nature of the bandgap of the SC-NC.


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Figure 2. UV-Vis-NIR spectra of the nanoparticles synthesized at 200 (a), 210 (b), 220 (c), 230 (d), 240 (e), and 250 °C (f). Figure 3 show TEM and HRTEM images of the SC-NCs synthesized at 250 °C. The images show round-like nanoparticles presenting average diameter of 21.5±8.0 nm (Figure 3a) and 14.5±5.5 nm (Figure 3c) obtained after 1 minute and 5 minutes of reaction, respectively. The decrease in the average size with the reaction time suggests that a process other than classical nucleation and growth or typical Ostwald ripening play a fundamental role in the formation of the nanoparticles. This process might be explained by the transition from Cu3SbS4 to CuSbS2 phase as revealed in the Raman spectra on Figure 1. The HRTEM image in Figure 3e (1 minute of reaction) shows the lattice interplanar distance of 3.0 Å, related to the (112) plane of the Cu3SbS4 phase (JCPDS 71-555). Figure 3f presents bright electron diffraction spots related to the (312) plane of the Cu3SbS4 phase (JCPDS 71-555).


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Figure 3. TEM images of the nanoparticles synthesized at 250 °C, after (a) 1 minute of reaction and (c) after 5 minutes of reaction and the respective histograms (b and d). HRTEM images of the samples synthesized at 250 °C (inset: Fast Fourier Transform of the planes), 1 minute of reaction (e) and SAED pattern (f). The XRD analyses of the purified samples (Figure 4) corroborate the results obtained from Raman spectroscopy (Figure 1). The presence of Cu3SbS4 is confirmed by the diffraction patterns at 23.7, 47.9, and 57.2° (marked with solid circle in Figure 4), related to the (112), (104), and (303) planes, respectively (JCPDS 71-555). These peaks are observed in the XRD of all samples synthesized after 1 minute of reaction and for all temperatures. In agreement to the Raman data (Figure 1), the CuSbS2 characteristic XRD peaks (marked with a solid star in Figure 4) arise for the samples synthesized with longer reaction times. Those peaks are located at 28.7,


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29.8, 49.8, and 52.0°, related to the plans (111), (013), (215), and (311), respectively (JCPDS 441417).

Figure 4. XRD diffraction patterns of the samples synthesized at 200 (a), 210 (b), 220 (c), 230 (d), 240 (e), and 250 °C (f). A series of experiments involving the same synthetic procedure but different reactant mixtures were performed to generate insights into the reaction mechanism. First, in order to evaluate if Sb2O3 was an intermediate or a by-product, a synthesis containing only SbCl3 and oleylamine was performed. The sample was analyzed by XRD and the results (Figure 5a) show that all of the SbCl3 was in fact converted to Sb2O3 (senarmontite). This is indicated by the peaks (solid square in Figure 5) at 13.7, 27.7, 32.1, 46.1, and 54.6°, related to the plans (111), (222), (040), (404), and (262), respectively (JCPDS 75-1565). The SbCl3 precursor is very reactive and, even in the presence of trace amounts of moisture or oxygen, it allows the formation of Sb2O3. Oxide


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formation due to the hydrolysis of SbCl3, induced by oleylamine, has already been reported.[36] Then, a synthesis using purified Sb2O3 as antimony precursor (rather than SbCl3) was performed. The results from XRD (Figure 5c) show the formation of Cu3SbS4 in the early stages of the reaction. In addition, in order to check if Cu3SbS4 was really converted to CuSbS2 as the reaction progresses, a synthesis using Cu3SbS4, SbCl3, and sulfur, without copper precursor was carried out. The XRD analysis after 9 minutes of reaction (Figure 5d) show the presence of CuSbS2, confirming the conversion from Cu3SbS4. Finally, in order to confirm the reduction strength of oleylamine at high temperatures, a mixture of oleylamine and Sb2O3 was heated to 250 °C for 10 minutes. The XRD analysis (Figure 5b) of the product of this reaction presented peaks at 28.7, 40.1, and 42.0°, representing the planes (012), (104), and (110), respectively, related to metallic antimony (JCPDS 85-1324). The formation of Sb0, confirms that metallic antimony is a byproduct when the synthesis of copper thioantimonate and thioantimonide are carried out at temperatures higher than 250 °C.

Figure 5. XRD diffraction patterns of control samples: (a) SbCl3-oleylamine solution at 200 °C, (b) Sb2O3-oleylamine solution heated to 250 °C, (c) synthesis of Cu3SbS4 using purified Sb2O3 as antimony precursor, and (d) synthesis of CuSbS2 departing from purified Cu3SbS4 nanoparticles.


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Considering the results from the control experiments in Figure 5, the formation of Cu3SbS4, CuSbS2 can be described by the simplified mechanism (non-balanced chemical equations) shown in Scheme 1.

Scheme 1. Reaction pathway for the formation of Cu3SbS4 and CuSbS2. Step 1 shows that SbCl3 is hydrolyzed in the presence of moisture. Notice that even after degassing, the presence of trace moisture can be enough to drive the hydrolysis.[37] The formation of Sb2O3 result in a milky white suspension in oleylamine. This kind of suspension has already been observed in the synthesis of thioantimony compounds, using antimony (III) acetate as well as antimony (III) trichloride as precursors.[23,38] CuCl and Sb2O3 precursors, followed by the injection of the sulfur solution, promotes step 2 (Scheme 1). Previous works on thioantimony compounds reported the formation of Cu3SbS4 from Sb3+ precursors, such as SbCl3 or antimony (III) acetate.[24-26] In these works, the formation of the famatinite phase is attributed to an excess of copper and sulfur in the precursor solution; however, no mechanism for the oxidation of Sb3+ was proposed. An explanation for the possible mechanism was obtained from studying geothermal waters, where it has been suggested that, in hydrothermal conditions, Sb3+ is oxidized by a combination of a sulfidic and basic environment.[39,40] In the present work, a basic chemical environment is provided by oleylamine, which in the presence of an excess of sulfur, creates the conditions for the oxidation from Sb3+ to


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Sb5+, forming SbS43-, as shown in reaction 2. This process leads to the formation of Cu3SbS4 shown on step 3 in scheme 1. The formation of Cu3SbS4, described in step 2 (scheme 1), results in a quick depletion of the free sulfur in the mixture. At those conditions, the reductive strength of oleylamine dominates and increases with the temperature,[22,41-43] leading to the transformation of the Cu3SbS4 into CuSbS2. Therefore, long exposure to oleylamine at high temperatures drives the Sb5+ reduction back to Sb3+, and in the presence of remaining Sb2O3 and sulfide, the formation of CuSbS2 occurs as described in step 4 of the Scheme 1. A mixture of Cu3SbS4 and CuSbS2 is not observed for long reaction times (see Figures 1 and 4) because the formation of Cu3SbS4 requires 4 times more sulfur than antimony. Once the sulfur excess is depleted, the Sb3+ is no longer converted to Sb5+ and Cu3SbS4 is no longer formed through step 3 (scheme 1). On the hand, the Sb5+ of famatinite, starts to be reduced by the oleylamine, and reacts in the presence of the remaining Sb2O3 and sulfide to convert it into CuSbS2 (step 4). Another potential pathway for the formation of CuSbS2 involves the dissolution of small Cu3SbS4 nanoparticles during the Ostwald ripening process. The dissolved Cu3SbS4 at high temperature might be reduced by oleylamine to Sb3+ and combine with sulfur excess to generate CuSbS2. This behavior is consistent to the smaller nanoparticles sizes after 5 minutes of reaction time when compared to those synthesized after 1 minute (Figure 1). This effect can be understood in terms of free energy for nucleation given by Equation 1:[44-46] ସ

∆‫ܩ‬ே = 4ߨ‫ ݎ‬ଶ ߛ + ߨ‫ ݎ‬ଷ ∆‫ܩ‬௩ (1) ଷ


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Where r is the particle radius, γ is the surface tension of the SC-NC, and ∆Gv represents the bulk free energy of the crystal. This energy is related to properties of the compound, as shown in Equation 2:[44-46]

∆‫ܩ‬௩ =

ି௞ಳ ்௟௡(ௌ) ௩೘

(2)

Here, kB is the Boltzmann constant, T is temperature, S is the solution supersaturation, and vm is the molar volume. According to equation 2, ∆Gv is inversely proportional to vm. ∆Gv is a negative quantity that helps to reduce the free energy of crystal formation, ∆GN. Therefore, the smaller molar volume will lead to more negative free energy ∆GN and more thermodynamically favorable crystals. The molar volume values for Cu3SbS4 and CuSbS2 are 90.43 cm3mol-1 and 49.87 cm3mol-1, respectively.[47] Consequently, from a simple thermodynamic point of view, CuSbS2 nanocrystals are more stable than Cu3SbS4. However, CuSbS2 is not formed at early stages of the reaction (short reaction times) due to the excess of sulfur in the presence of oleylamine, which creates the conditions for the oxidation of Sb(III) to Sb(V). This simple thermodynamic argument is consistent with the predominance of CuSbS2 at longer reaction times (5 and 10 minutes; see Figures 1 and 4). Figure 6 compares previous reports on oleylamine-assisted syntheses of CuxSbySz, and shows a clear trend for the formation of Cu3SbS4 (Sb5+) at lower temperatures and/or shorter reaction times. Either CuSbS2 or Cu3SbS3 (Sb3+) are preferentially formed at higher temperatures and/or longer reaction times (Fig. 6). All of the works selected for Figure 6 used a Sb3+ source precursor, such as SbCl3 or antimony (III) acetate.


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Figure 6. (a) Copper antimony sulfides phases formed in oleylamine-based solvothermal synthesis with different conditions and (b) Cu3SbS4 and CuSbS2 formed in this work. It is possible to observe the same trend, formation of Cu3SbS4 nanoparticles at lower temperature or shorter reaction time and the formation of CuSbS2 at higher temperature or longer reaction time, in this work (Figure 6b). Therefore, the mechanisms suggested here appear consistent with the formation of the different copper antimony sulfide species in oleylamine-based syntheses throughout the literature. The robustness of the proposed mechanism is further validated by analyzing the results from refs. 52 and 53. John and coworkers[48] reported the formation of Cu3SbS4 at lower temperatures (200 to 220 °C) when compared to the temperatures required to form CuSbS2 (230 to 240 °C), in a solvothermal synthesis assisted by polyvinylpyrrolidone. In the same way, Shi and coworkers[49] reported an ethylenediamine-assisted solvothermal synthesis, where Cu3SbS4 was formed at 200 °C, while CuSbS2 was formed at 230 °C, with reaction time of 20 hours in both cases. CONCLUSION


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In this work we provide a mechanism for the synthesis of Cu3SbS4 and CuSbS2 assisted by oleylamine. The results are compared with several literature reports, showing a good level of consistence on the explanation of the formation of the different phases of copper antimony sulfides. By probing and understanding the role of antimony during the formation process, we were able to propose a mechanism that can be useful to further tune the composition of antimony sulfosalts, as well as arsenic sulfosalts, which have a very similar chemical behaviour as antimony. The results presented in this work can allow the design of materials with precisely controlled stoichiometry and crystal structure, leading to a greater control of their bandgaps. ASSOCIATED CONTENT Supporting Information. Tauc plot and optical bandgap calculations. This material is available free of charge on Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources


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We thank scholarships from CAPES and funding from CNPq (408182/2016-4). The Canadian portion of the work was funded by a Discovery Grant from NSERC and equipment grant from CFI. ACKNOWLEDGMENT FB thanks the Emerging Leaders of the Americas Program for a visiting student fellowship. The authors thank CNANO, CMM and CAMTEC for the facilities. REFERENCES (1) Kong, Y. L.; Tamargo, I. A.; Kim, H.; Johnson, B. N.; Gupta M. K.; Koh, T-W.; Chin, H-A.; Steingart, D. A.; Rand, B. P.; Mcalpine, M. C. 3D Printed Quantum Dot Light-Emitting Diodes. Nano Lett. 2014, 14 (12), 7017–7023. (2) Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Solution-processed, high-performance light-emitting diodes based on quantum dots. Nature, 2014, 515, 96-99. (3) Song, W-S. and Yang, H. Efficient White-Light-Emitting Diodes Fabricated from Highly Fluorescent Copper Indium Sulfide Core/Shell Quantum Dots. Chem. Mater., 2012, 24, 19611967. (4) Alivisatos, A. P.; Bruchez Jr., M.; Moronne, M.; Gin, P.; Weiss, S. Semiconductor Nanocrystals as Fluorescent Biological Labels. Science, 1998, 281, 2013-2016. (5) Hong, Z-H.; Lv, C.; Liu, A-A.; Liu, S-L.; Sun, E-Z.; Zhang, Z-L.; Lei, A-W.; Pang, D-W. Clicking Hydrazine and Aldehyde: The Way to Labeling of Viruses with Quantum Dots. ACS Nano, 2015, 9 (12), 11750-11760.


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For Table of Contents Use Only

Uncovering the mechanism for the formation of copper thioantimonate (SbV) nanoparticles and its transition to thioantimonide (SbIII) Fábio Baum,a Tatiane Pretto,b Alexandre G. Brolo,c* Marcos José Leite Santosa,b*

A synthetic and mechanistic study of the formation of copper thioantimonates (Cu3SbS4) and thioantimonides (CuSbS2) nanoparticles is presented. The initial sulfur excess led to formation of Cu3SbS4. As the reaction progressed, the antimony (V) of the Cu3SbS4 was reduced to antimony (III), forming CuSbS2. The formation mechanism unveiled here provides important insights towards the synthesis of analogous materials.


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Uncovering the mechanism for the formation of copper

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