Synthesis of eco-friendly ternary sulfide nanocrystals using a new sulfur–decene reagent in hydrocarbons

A new, highly reactive sulfur precursor was synthesized by dissolving elemental sulfur in 1-decene at an elevated temperature of 170 °C in a pressure tube, enabling the preparation of semiconductor sulfide nanocrystals (NCs) with diverse compositions and morphologies. By application of this new reagent, we demonstrated the preparation of eco-friendly ternary sulfide ABS₂ NCs (A = Cu and Ag; B = Ga and In). The chemical composition of this sulfur reagent was analyzed by spectroscopic methods, and reactive species were revealed. The dependence of the morphology, size, and optical properties of CuInS₂ NCs on the applied reagents and solvents was studied in detail. Hexadecane was identified as the solvent providing the narrowest distribution among hydrocarbons. Ag₉GaS₆ colloidal NCs were obtained for the first time by applying the new sulfur precursor and were fully characterized. All nanoparticles were characterized by optical spectroscopy, XRD, TEM, EDX, XPS, Raman spectroscopy, and IR. Detailed Fourier-transform infrared (FT-IR) analysis of ABS₂ NCs revealed the mixed nature of the ligand shell.

1 Introduction

Colloidal semiconductor nanocrystals (NCs) are a rapidly developing area of research in material science with various applications; some of them, such as QLED displays, are now widely adopted in consumer electronics. Another important technology, namely, nanocrystal-based short-wave infrared cameras, was recently introduced to the market (Sharma et al., 2024).

The chalcopyrite-type semiconductors ABS₂ (A = Cu and Ag; B = Ga and In) possess band gaps in the range from 1.53 eV for CuInS₂ to 2.7 eV for AgGaS₂ in bulk (Woods-Robinson et al., 2020). Metal-indium-sulfides (AInS₂) have narrower band gaps than their gallium counterparts (AGaS₂). It makes nanocrystals based on these materials favorable for applications in UV, visible, and near-IR ranges of the spectrum. Copper and indium are much less toxic to humans than cadmium and lead (Nakajima et al., 2008). These properties make them possible eco-friendly alternatives to the ecologically problematic cadmium selenide quantum dots (Liu et al., 2021). Ternary chalcogenide nanocrystals of the ABS₂-type exhibit both oxidative stability and very high hydrolysis stability, which make them very promising industrial nanomaterials (Yang et al., 2016).

These colloidal NCs are used in light emitting diodes and and solar cells (Pan et al., 2014) because of their optical properties (Chuang et al., 2014). The lower toxicity of ABS₂ nanoparticles makes them preferred materials for bioimaging in cancer therapy (Yong et al., 2010; Song et al., 2016) and for biosensing applications (Liu H et al., 2015). High oxidation stability make these nanoparticles advantageous for application as photocatalysts for CO₂ reduction (Zhang et al., 2023; Rahman and Khan, 2021) and wastewater treatment (Olatunde et al., 2025; Ganguly et al., 2020). Recently, colloidal CuInS₂ NCs were applied for the direct photoreduction of CO₂ to CO in pure water. In combination with a co-porphyrin catalyst, a CO turnover number over 80,000 was achieved (Arcudi et al., 2021). An AgInS₂ NC-based catalyst was used as a highly efficient catalyst for hydrogen production (Yu et al., 2022). Photocatalytic N₂ fixation was reported recently using a CuInS₂ NC-based catalyst (Bariki et al., 2023). Bulk ABS₂ materials are known for their interesting nonlinear-optical properties; the ABS₂ nanocrystals in colloidal form offer additional opportunities in processing and applications (Boyd et al., 1971). At the moment, copper indium sulfide is the most explored nanomaterial among ABS₂-type nanocrystals (Rahman et al., 2024). The important properties of ABS₂ materials are summarized in Table 1.

Table 1

Table 1. Selected properties of ABS₂ semiconductors in bulk.

A wide variety of synthetic approaches for the production of ternary colloidal chalcogenide nanocrystals have been studied and extensively reviewed (Coughlan et al., 2017; Rahman and Khan, 2021; Xing and Chen, 2025). The solvothermal synthesis is the most popular approach for this type of nanocrystal. The preparation of the chalcopyrite-type ternary NCs is a challenging task due to the different nature and reactivity of the applied metal ions. In³⁺ and Ga³⁺ are hard Lewis acids, whereas both Cu⁺ and Ag⁺ are soft in character. The sulfide anion is also a soft Lewis acid. It means that binary copper or silver sulfide will form first if the reactivity of the applied metal-precursors is not adjusted (Kolny-Olesiak and Weller, 2013).

The chalcogenide nanocrystals were mostly synthesized by the hot-injection method using various chalcogen precursors (Mishra et al., 2016). The synthesis of chalcopyrite nanocrystals is dominated by the pyrolysis of mercaptans (Lu et al., 2022). These foul-smelling and moderately toxic reagents require high temperatures over 220 °C for the synthesis of sulfide nanocrystals (Zhong et al., 2010). In addition, mercaptans are strongly bonding ligands that hinder ligand exchange with other ligand types in solution or in thin films, thus limiting possible applications of synthesized nanoparticles, which has motivated interest in ternary sulfide ABS₂ (A = Cu and Ag; B = Ga and In) nanocrystals; the series of sulfur precursors from decene-1 was prepared, and their composition was investigated.

Finally, we developed a sulfur precursor, which was then applied for the preparation of a wide range of colloidal ternary sulfide nanocrystals, including previously unknown colloidal Ag₉GaS₆ NCs. This new reagent is based on commodity chemicals, namely, elemental sulfur and decene-1, which makes the preparation of nanocrystals economically feasible. In addition to its economic advantages, the new sulfur precursor demonstrates moderate stability toward both moisture and atmospheric oxygen, representing a notable improvement over bis(trimethylsilyl)sulfide, a widely used but highly air- and moisture-sensitive reagent in sulfide NC synthesis. Taken together, these features make the developed precursor a practical and efficient alternative for the scalable preparation of ternary sulfide nanomaterials.

2 Materials and methods

2.1 Chemicals

The following solvents and chemicals were used without additional purification: acetone (99%, Component-Reaktiv), copper (I) chloride (99%, Lanhit), decane (99%, Component-Reaktiv), decene-1 (90%, Vekton), gallium (99.99%, Lanhit), hexadecane (99%, Component-Reaktiv), indium (99.99%, Lanhit), indium chloride (99%, Lanhit), indium iodide (99%, Lanhit), sulfur (99.999%, Acros), silver nitrate (reagent-grade, Vekton), hexane (99% HPLC-grade, Macron Fine Chemicals), propanol-2 (99%, Component-Reaktiv), and methanol (99%, reagent-grade, Khimmed). Oleylamine (80%–90%, Acros) was dried at 110 °C at reduced pressure (5 mbar) for 2 h. Gallium and indium stearates were prepared from elemental gallium and indium according to the method described in the literature (Dou and Ng, 2016). Silver stearate was prepared from silver nitrate (Malik et al., 1971).

2.2 Characterization

The optical properties of the as-synthesized colloidal nanocrystals were studied by ultraviolet–visible (UV–VIS)–near infrared (NIR) spectroscopy. The UV–VIS spectra were recorded over the range of 200 nm–2,500 nm using a UV–VIS–NIR spectrophotometer (V-770, JASCO). Emission spectra were recorded using a spectrofluorimeter (V-770, JASCO). The crystal structure was identified using a Drawell DW-XRD-2700A powder diffractometer (Cu Kα = 1.5406 Å, 40 kV, 40 mA, graphite monochromator, closed proportional detector). The MDI Jade 6.5 software package was used for XRD phase analysis. The chemical compositions were explored by Fourier-transform infrared (FT-IR) spectroscopy (Spectrum 100, PerkinElmer). ¹H NMR spectra were recorded using a 500 MHz Varian Unity Inova Spectrometer. Chemical shifts (ppm) are given relative to the solvent. The morphological and structural features of the synthesized nanoparticles were investigated using a transmission electron microscope (TEM, JEM-2100, JEOL) operated at an accelerating voltage of 200 kV and equipped with an energy-dispersive X-ray (EDX) spectrometer (X-MaxN, Oxford Instruments). The Raman spectra were determined using a DXR3 Raman Microscope (Thermo Fisher Scientific). X-ray photoelectron spectra (XPS) of samples were obtained using a hemispherical electron analyzer PHOIBOS 150 with excitation by monochromatized Al K_α radiation (photon energy 1486.61 eV and resolution ΔE = 0.2 eV), which is located at the NanoPES beamline of Kurchatov Synchrotron–Neutron Research Complex (National Research Center “Kurchatov Institute”) (Lebedev et al., 2021). Samples were fixed on a sample holder and then transferred to the spectrometer’s vacuum chamber, which had a base pressure of 3·× 10⁻⁹ mbar. All spectra were measured in the constant transmission energy mode with energies of 120 and 60 eV for the overview spectra and fine structure of individual lines, respectively. CasaXPS software was used to analyze the experimental data and decompose the lines into components (Fairley et al., 2021). All samples for XPS analysis were prepared by drop-casting nanoparticle colloidal solutions onto silicon wafers, and the measurements were performed directly on these samples without subsequent sputter etching using argon/oxygen plasma.

2.2.1 Preparation of the sulfur precursor

An elemental sulfur sample (4 mmol, 0.128 g) was dissolved in 10 mL of decene-1 (7.4 g, 52 mmol) by stirring for 1 h in a thick-walled pressure tube at 170 °С. The resultant yellow, 0.4 M solution of the sulfur precursor could be stored or applied immediately for the synthesis of nanocrystals.

2.2.2 Synthesis of CuInS₂ colloidal nanocrystals

In the typical synthesis process, copper chloride (6.0 mg, 0.06 mmol), indium stearate (57.9 mg, 0.06 mmol), oleylamine (0.6 mL, 3.6 mmol), and decane (2 mL) were placed into a three-neck round-bottom flask in an argon atmosphere. The resultant mixture was stirred and heated to 110 °C for 1 h under an argon atmosphere until a homogeneous solution was formed. Then, the resultant solution was diluted with decane (18 mL) and heated up to 150 °C. A 0.4 M solution of sulfur in decene-1 (7.2 mL) was swiftly injected using a syringe. The reaction mixture was stirred at 150 °C for 90 min under continuous Ar flow. The reaction was terminated by placing the reaction flask in an ice bath.

A measure of 4.8 mL of the isopropanol: methanol mixture (2:1) was added to every 6 mL of the resulting solution. Then, it was centrifuged to isolate the precipitate. After that, the resulting precipitate was redispersed in 2 mL of hexane, and 2.8 mL of the isopropanol: methanol mixture (2:1) was added to the resulting solution, which was then centrifuged again. The resulting precipitate was dried under an Ar flow and redispersed in 0.5 mL of tetrachloroethylene (TCE). Finally, the resulting solution was filtered, diluted to 1.5 mL by adding TCE, and characterized using a JASCO V-770 spectrophotometer and a transmission electron microscope.

2.2.3 Synthesis of CuGaS₂ colloidal nanocrystals

In the typical synthesis process, copper chloride (6.0 mg, 0.06 mmol), gallium stearate (55.2 mg, 0.06 mmol), oleylamine (0.6 mL, 3.6 mmol), and hexadecane (2 mL) were placed into a three-neck round-bottom flask in an argon atmosphere. The resultant mixture was stirred and heated to 110 °C for 1 h under an argon atmosphere until a homogeneous solution was formed. Then, the resultant solution was diluted with hexadecane (18 mL) and heated up to 190 °C. A 0.4 M solution of sulfur in decene-1 (12 mL) was swiftly injected using a syringe. The reaction mixture was stirred at 190 °C for 90 min under continuous Ar flow. The reaction was terminated by placing the reaction flask in an ice bath.

A measure of 5.2 mL of the isopropanol: methanol mixture (4:1) was added to every 5 mL of the resultant solution. Then, it was centrifuged to isolate the precipitate. After that, the resulting precipitate was redispersed in 2 mL of hexane, and 3 mL of the isopropanol: methanol mixture (4:1) was added to the resultant solution, which was then centrifuged again. The resulting precipitate was dried under an Ar flow and redispersed in 0.5 mL of tetrachloroethylene. Finally, the resultant solution was filtered, diluted to 1.5 mL by adding TCE, and characterized using a JASCO V-770 spectrophotometer and a transmission electron microscope.

2.2.4 Synthesis of AgInS₂ colloidal nanocrystals

In the typical synthesis process, silver stearate (23.5 mg, 0.06 mmol), indium stearate (57.9 mg, 0.06 mmol), oleylamine (1.2 mL, 3.6 mmol), and decane (2 mL) were placed into a three-neck round-bottom flask in an argon atmosphere. The resultant mixture was stirred and heated to 110 °C for 1 h under an argon atmosphere until a homogeneous solution was formed. Then, the resultant solution was diluted with decane (18 mL) and heated up to 150 °C. A 0.4 M solution of sulfur in decene-1 (12 mL) was swiftly injected using a syringe. The reaction mixture was stirred at 150 °C for 90 min under continuous Ar flow. The reaction was terminated by placing the reaction flask in an ice bath.

A measure of 3.6 mL of the isopropanol: methanol mixture (2:1) was added to every 6 mL of the resultant solution. Then, it was centrifuged to isolate the precipitate. After that, the resultant precipitate was redispersed in 2 mL of hexane, and 2.8 mL of the isopropanol: methanol mixture (2:1) was added to the resultant solution, which was then centrifuged again. The resulting precipitate was dried under an Ar flow and redispersed in 0.5 mL of tetrachloroethylene. Finally, the resultant solution was filtered, diluted to 1.5 mL by adding TCE, and characterized using a JASCO V-770 spectrophotometer and a transmission electron microscope.

2.2.5 Synthesis of Ag₉GaS₆ colloidal nanocrystals

In the typical synthesis process, silver stearate (23.5 mg, 0.06 mmol), gallium stearate (55.2 mg, 0.06 mmol), oleylamine (1.2 mL, 3.6 mmol), and hexadecane (2 mL) were placed into a three-neck round-bottom flask in an argon atmosphere. The resultant mixture was stirred and heated to 110 °C for 1 h under an argon atmosphere until a homogeneous solution was formed. Then, the resultant solution was diluted with hexadecane (18 mL) and heated up to 150 °C. A 0.4 M solution of sulfur in decene-1 (12 mL) was swiftly injected using a syringe. The reaction mixture was stirred at 190 °C for 90 min under continuous Ar flow. The reaction was terminated by placing the reaction flask in an ice bath.

A measure of 4.8 mL of the isopropanol: methanol mixture (2:1) was added to every 6 mL of the resultant solution. Then, it was centrifuged to isolate the precipitate. After that, the resulting precipitate was redispersed in 2 mL of hexane, and 2.8 mL of the isopropanol: methanol mixture (2:1) was added to the resulting solution, which was then centrifuged again. The resulting precipitate was dried under an Ar flow and redispersed in 0.5 mL of TCE. Finally, the resultant solution was filtered, diluted to 1.5 mL by adding TCE, and characterized using a JASCO V-770 spectrophotometer and a transmission electron microscope.

3 Results and discussion

3.1 Sulfur precursor composition

Within the scope of our research on chalcogenide precursors for colloidal semiconductor nanocrystal synthesis (Shuklov et al., 2023; Shuklov et al., 2020), we developed selenium precursor by dissolving elemental gray selenium in decene-1 (Shuklov et al., 2025). We decided to analyze a similar reagent as a sulfur precursor. The reaction of unsaturated hydrocarbons with elemental sulfur at elevated temperatures is mostly known as vulcanization of rubber and is already well explored. The thermally initiated reaction of sulfur with terminal alkenes is known (Onose et al., 2022) but has been less extensively studied. Depending on the substrate and reaction conditions, the reaction can proceed either as an addition to the double bond (Nguyen, 2017) or an insertion into the C–H bond of the methylene group adjacent to the double bond (Cataldo, 2023), which is also known as a “cure site” in rubber chemistry. Precursors of sulfur were obtained by dissolution of elemental sulfur in octadecene-1 and applied in the synthesis of several colloidal NCs, such as CdS (Li et al., 2011) and PbS (McPhail and Weiss, 2014).

We found that the behavior of the reagent obtained by dissolving sulfur in decene-1 depends on the reaction temperature. The reactions at temperatures close to or above the boiling point of decene-1 were performed in thick-walled pressure tubes. In addition, performing the reactions in closed vessels could influence the products since the possible gaseous products, such as H₂S, could not escape the reaction mixture and may undergo further reactions. The dissolution of S₈ at 150 °C yields a pale yellow solution that decomposes within an hour at room temperature, providing a precipitate of beta-sulfur (precursor A). Increasing the reaction temperature up to 170 °C leads to the formation of a darker-colored solution and the irreversible dissolution of sulfur (precursor B). Both solutions could be applied as sulfur precursors; however, due to easier handling, the solution of sulfur obtained at 170 °C was applied further.

The nature of the prepared sulfur precursor B was analyzed via NMR spectroscopy, GC–MS, and optical spectroscopy. The absorption spectra indicated that the absorption red edge of sulfur solutions is farther in the solution obtained at 170 °C than in the one obtained at 150 °C (Supplementary Figure S1). Sulfur could be dissolved in a concentration up to 2.4 M, forming red–brown solutions. ¹H NMR of the 0.4 M solution of sulfur indicates only minor changes in the spectrum compared to the starting decene-1. Intensities of new signals from sulfur are comparable with the intensities of impurities. The multiplets at 2.7 ppm–3.2 ppm could be attributed to the sulfur-substituted alkyl chain—a product of insertion. The ratio of signals of olefinic hydrogens to the methyl group did not change significantly. Nevertheless, only one-third of the unreacted alkene could be distilled from the crude reagent. H₂S was not detected in the reaction mixture. All these observations indicated the formation of high-boiling sulfur derivatives with long sulfur chains and azeotropic mixtures involving them. ¹H NMR of the more concentrated 1.2 M solution shows some excess of methyl protons over olefinic protons. The H,H-COSY revealed that the signals at 3.0 and 3.2 have cross-peaks, indicating the formation of 1,2-sulfur-substituted derivatives (Supplementary Figure S2). Both ¹H NMR and GC–MS show that decene-1 remains the main constituent of the sulfur-precursor for the solution from 0.4 M up to 1.2 M sulfur. Two products of sulfur addition to decene-1 were identified by GC–MS with an FID detector, namely, decyl sulfide and decyl disulfide (Supplementary Figure S3). Longer sulfides are probably unable to survive a GC-column at temperatures over 200 °C. A higher concentration of sulfur (2.4 M) produced a different pattern in ¹H NMR and resulted in a more viscous solution.

3.2 ABS₂ NC synthesis

The CuInS₂ nanocrystals were prepared by the reaction of copper chloride and indium stearate solution in oleylamine with the sulfur precursor at 150 °C. Indium stearate was chosen as the starting material due to its low hygroscopicity compared to indium halides. It was also analyzed by powder XRD (Supplementary Figure S4) to simplify further analysis of various starting material batches. Both indium and gallium stearates are easily available from gallium and indium metals and stearic acid.

The reaction mixture was diluted with alkanes, namely, decane and hexadecane, to reduce agglomeration and achieve a narrower size distribution. For screening the medium for nanoparticle synthesis, several factors should be considered. First, the media should efficiently dissolve the reactants, even at high concentrations. Second, the media must have low viscosity in order to obtain homogeneous reaction mixtures at the initial stages of synthesis. The solvent should be easily removable to obtain only nanocrystals as a product. The two tested hydrocarbons have different viscosity values, which could have impacted the outcome of NC synthesis. Decane has a dynamic viscosity of 0.850 mPa/s, and hexadecane has a dynamic viscosity of 3.03 mPa/s.

CuInS₂ nanoparticles with a mean size of 5.5 nm were obtained in decane from indium stearate with 0.4 M sulfur precursor B (Figure 1). Application of hexadecane provided slightly smaller NCs with a mean size of 4.8 nm and better size deviation (SD 0.72). Application of a lower temperature to 0.4 М precursor A with the same indium stearate yielded much larger nanocrystals (19 nm) with a poor size distribution (SD 3.3). Precursor B provides smaller NCs than precursor A under the same reaction conditions. The prepared NCs with 0.4 M precursors A and B exhibited a maximum emission at 540 nm with all the tested indium precursors (Figure 2). As shown in Figure 1C, the XRD pattern of the obtained sample corresponds to the literature data for tetragonal CuInS₂ with I-42d space group (JCPDS Card: 38–777), and no extraneous diffraction reflections were found. Similar behavior was reported earlier for CuInS₂ NCs prepared by solvothermal synthesis (Li and Teng, 2010). The XRD spectrum clearly ruled out the presence of binary chalcogenide phases such as Cu₂S, CuS, and In₂S₃, indicating that the obtained diffraction peaks correspond exclusively to the ternary compound (Borkovska et al., 2015). The literature data for binary sulfides are presented in Figure 1C for comparison. According to the Scherrer equation, the nanocrystallites have an average diameter of 2.7 ± 1 nm, indicating that part of the nanoparticle material may exist in an amorphous form.

Figure 1

Figure 1. CuInS₂ NCs prepared from indium stearate in hexadecane: (A) TEM image; (B) size-distribution diagram; (C) XRD pattern; (D) absorption spectra; (E) Tauc plot; (F) Raman spectrum measured with 532-nm excitation wavelength.

Figure 2

Figure 2. CuInS₂ NCs synthesized at 150 °C in decane by varying the reaction medium: (A) absorption spectra in the visible range of 400 nm–1,000 nm and (B) emission spectra.

A Tauc plot for as-synthesized CuInS₂ NCs is depicted in Figure 1E. The band gap E_bg = 2.73 eV was determined for these NCs. In the Raman spectrum, a broad peak with a maximum at 290 cm^-1 could be deconvoluted into three components centered at 260, 290, and 320 cm⁻¹ (Figure 1F). Such a pattern is reported for CuInS₂ nanoparticles and thin films (Izquierdo-Roca et al., 2011). The signal at 290 cm^-1 corresponds to the typical A1 mode of CuInS₂ chalcopyrite. The Raman spectrum also verifies the absence of CuS and the Cu_xS phases, and these peaks are typically located at approximately 470 cm⁻¹ and 492 cm⁻¹.

The execution of the NC synthesis with both indium stearate and chloride in hexadecane with precursor B under the same reaction conditions yields slightly smaller NCs with better size distribution than in the case of decane (Table 2, runs 1–2 and 6–7). As usual, temperature has a positive impact on the size of synthesized NCs and a negative impact on the size deviation. Therefore, the mean size of CuInS₂ NCs increases from 6.0 nm at 130 °C to 8.9 nm at 190 °C with indium stearate (runs 4 and 8). Indium chloride provides smaller NCs than those with indium stearate.

Table 2

Table 2. Comparison of S-precursors in the synthesis of AgInS₂ NCs under various reaction conditions.

The influence of the metal ratio on the outcome of the nanoparticle synthesis was analyzed. A double excess of both indium and copper has a negative impact on the outcome of CuInS₂ NC synthesis. The excess of indium leads to amorphous, poorly formed nanoparticles. Excess copper leads to the formation of NCs with various morphologies and sizes over 10 nm. The application of concentrated S-precursor B results in CuInS₂ samples with an emission maximum at 432 nm.

The experimental chemical composition of NCs obtained with precursor B was established by EDX analysis: Cu, 22.1 wt%; In, 44.0 wt%; and S, 34.2 wt% (Supplementary Figure S1). The calculated elemental composition for CuInS₂ is Cu, 26.20%; In, 47.35%; and S, 26.45%. The calculated composition indicates an excess of sulfur in the synthesized nanocrystals caused by applying an excess of sulfur precursor.

The CuInS₂ nanocrystals prepared applying precursor A were successfully applied for the fabrication of a photoresistor by ligand exchange with thiocyanate. It exhibited a photo-response to a 405 laser (Shuklov et al., 2025).

Additional analysis of the elemental composition and electron states was performed by X-ray photoelectron spectroscopy (XPS). Only copper, indium, sulfur, carbon, oxygen, and nitrogen signals could be observed in this spectrum, indicating high purity of the prepared nanocrystals (Figure 3). Carbon, nitrogen, and oxygen were present due to the organic ligand shell and atmospheric contamination. The quantification results showed the following elemental composition by weight: Cu, 8.85%; In, 30.28%; S, 60.87%. The copper-to-indium ratio is stoichiometric 1:2. The XPS-analysis also supports the sulfur-rich character of the nanoparticle surface. Since the studied nanoparticles are several nanometers in size and possess a long-chain organic ligand shell with an approximate thickness of 1 nm, the elemental composition results differ from those of EDX.

Figure 3

Figure 3. XPS CuInS₂ NCs. (A) XPS survey spectrum and high-resolution XPS scan spectra over the (B) Cu 2p region, (C) In 3d region, and (D) S 2p region.

Both the copper 3d and indium 3d signals could be perfectly fitted using a Gaussian (50%)–Lorentzian (50%) profile, indicating a single oxidation state for both metals. Indium 3d_5/2 and In 3d_3/2 peaks are observed at 444.9 eV and 452.4 eV, respectively. The core level of the Cu 2p spectrum is split into Cu 2p_3/2 (931.5 eV) and Cu 2p_1/2 (952.5 eV) (Figure 3B). The sulfur high-resolution spectrum exhibits a slightly broadened peak centered at 162.5 eV, which results from the overlapping S 2p_3/2 and S 2p_1/2 signals. This allows us to exclude oxidation of our samples to sulfates as these signals are normally observed at higher energies (167 eV–169 eV).

The developed procedure for the synthesis of CuInS₂ was extended to the synthesis of other MInS₂-type nanocrystals. AgInS₂ NCs were synthesized by a similar procedure at 150 °C. Decane was applied as a reaction medium, and stearates of silver and indium were applied as reagents. It should be noted that the application of halides of both silver and indium is impossible due to the low solubility of silver halides in the reaction mixture. TEM microphotography revealed round and pear-shaped nanoparticles of AgInS₂ (Figure 4). Similar morphology was reported for AgInSe₂ colloidal nanocrystals prepared by the hydrothermal method (Liu Z et al., 2015), but it is the first example of pear-shaped morphology for AgInS₂ NCs. These AgInS₂ particles have an average length of approximately 17 nm with an average width of approximately 8.5 nm. The average ratio of sides in NCs is approximately 2 (Figures 4C, D). The X-ray diffraction pattern confirms the chalcopyrite structure of the AgInS₂ NCs. The observed diffraction peaks belong to the AgInS₂ orthorhombic phase with cell parameters a = 7.001Å, b = 8.278 Å, and c = 6.698 Å (literature data) and the Pna2₁ (33) space group (JCPDS Card: 25–1328). The presence of other phases, such as hexagonal AgInS₂, with space group R $\overline{3}$ m (166) (JCPDS Card: 22–1329) and cubic AgIn₅S₈ with space group Fd $\overline{3}$ m (227) (JCPDS Card: 25–1329), was not detected. We also note that the AgInS₂ phase could be observed in a natural mineral, namely, laforêtite (JCPDS Card: 25–1330). No peaks of impurities such as Ag₂S or any other material were observed. The polycrystalline nature of the synthesized material was indicated by broad peaks. As shown in Figure 4B, there is a shift to the region of smaller angles of all X-ray maxima, relative to the literature data. As is known, shifts of the specimen surface lower than the ideal value move the peaks to lower angles (Kaduk et al., 2021); this was also observed in our case since the sample was obtained in an extremely small quantity, making it impossible to position it ideally in the cuvette. At the same time, it is known that for many nanomaterials, an increase in lattice parameters (Deshpande et al., 2005) is observed with a decrease in size. As was shown, according to TEM data, the particle size is significantly small (17 nm in length and 8.5 nm in width on average); therefore, this size effect cannot be excluded. Taking into account all of the above, we did not refine the lattice parameters, since the obtained data cannot be unambiguously interpreted.

Figure 4

Figure 4. AgInS₂ NCs prepared at 150 °C. (A) TEM image; (B) XRD; (C) longitudinal size-distribution diagram; (D) transversal size-distribution diagram; (E) absorption spectra of AgInS₂ NCs, synthesizes at 40 and 90 min.

EDX analysis indicates a 1.02:1 Ag: In ratio in nanocrystals with a total composition of In 40.0 %wt, Ag 38.6%, and S 21.4% (Figure 5). EDX element distribution maps show uniform distribution of Ag, In, and S, indicating that the obtained NCs are all ternary sulfides without the possible impurity of silver sulfide. The absence of Ag₂S as a byproduct was confirmed by XRD since the corresponding phase was not detected. A Tauc plot for as-synthesized AgInS₂ NCs is shown in Figure 5F. The band gap E_bg = 1.96 eV was determined for these NCs. The Raman spectrum shown in Figure 5G closely resembles the earlier reported spectra of AgInS₂ samples (Azhniuk et al., 2023).

Figures 5

Figures 5. AgInS₂ NCs. (A) EDX spectrum; (B) TEM image; (C) Ag distribution map; (D) In distribution map; (E) S distribution map; (F) Tauc plot; (G) Raman spectrum.

In general, reaction temperatures below 160 °C for the synthesis of CuInS₂ and AgInS₂ NCs could completely prevent the evolution of hydrogen sulfide as a sulfur precursor, as was previously suggested for the octadecene–sulfur system (McPhail and Weiss, 2014). In the case of decene–sulfur precursor B, the formed organic oligosulfides are expected to serve as the sulfur source.

XPS spectra were used to characterize the nanoparticle composition more accurately, as shown in Figure 6.

Figure 6

Figure 6. XPS AgInS₂ NCs. (A) XPS survey spectrum and high-resolution XPS scan spectra over the (B) Ag 3d region, (C) In 3d region, and (D) S 2p region.

Only silver, indium, sulfur, carbon, oxygen, and nitrogen signals could be observed in this spectrum, indicating the contamination-free nature of the prepared nanocrystals (Figure 3). Carbon, nitrogen, and oxygen are present due to the organic ligand shell and adsorbed carbon dioxide from the atmosphere. The following elemental composition by weight was established from the XPS spectrum: Ag, 25.1%; In, 21.57%; S, 53.33%. The silver: indium: sulfur ratio was 1.2:1:9. These results additionally indicate that the nanoparticle surface is enriched in sulfur. Considering that the particles are only a few nanometers in diameter and are coated with a ∼1-nm-thick organic ligand shell, deviations from the EDX-derived compositions are expected.

Both silver 3d and indium 3d signals could be perfectly fitted by the Gaussian (50%)–Lorentzian (50%) profile, indicating a single oxidation state for both metals. Indium 3d_5/2 and In 3d_3/2 peaks are observed at 443.6 eV and 451.1 eV, respectively. The Ag 3d spectrum is split into Ag 3d_5/2 (366.6 eV) and Ag 3d_3/2 (372.5 eV) (Figure 3B). The high-resolution sulfur spectrum exhibits a slightly broadened peak centered at 162.5 eV, resulting from the overlap of the S 2p_3/2 and S 2p_1/2 signals. This allows us to exclude oxidation of our samples to sulfates or sulfites since these signals are normally observed at higher energies (167 eV–169 eV).

Silver indium sulfide NCs were applied for the preparation of a photosensitive layer in a photoresistor. This photodetector, based on a thin film of AgInS₂ NCs, showed photoresponse under on/off light radiation with an illumination wavelength of 405 nm (Supplementary Figure S11).

The successful application of the new sulfur precursor inspired us to investigate the synthesis of gallium analogs of the nanocrystals. Initial screening revealed that gallium MGaS₂ nanocrystals do not form at temperatures as high as 150 °C. Further increasing the reaction temperature requires higher boiling solvents; therefore, decane was replaced by hexadecane as the reaction medium. The initial screening revealed that temperatures of 190 °C or higher are necessary for the formation of MGaS₂ nanocrystals. CuGaS₂ NCs were obtained in hexadecane as a reaction medium at 220 °C. TEM revealed a mixture of nanocrystals of two morphologies: smaller ones have spherical and rhombical shapes, and larger particles are rhombical with a mean diameter of approximately 13 nm (Figures 7A,B). The X-ray diffraction pattern of the synthesized nanocrystals confirms the chalcopyrite structure of the mineral gallite CuGaS₂ (Figure 7C). Diffraction reflexes from the [112], [204], and [312] planes (indicated on Figure 7C) correspond to the tetragonal chalcopyrite structure, space group I-42d (122). Size analysis from XRD, calculated using the Scherrer equation, provides an average nanocrystallite diameter of 7.3 ± 0.9 nm, implying that a part of the nanoparticle material exists in an amorphous state.

Figure 7

Figure 7. CuGaS₂ NCs. (A) TEM image; (B) size-distribution diagram; (C) XRD-pattern; (D) absorption spectra synthesized at 90 min: (E) Tauc plot; (F) Raman spectrum.

The shift in the position of the X-ray maxima for the CuGaS₂ sample is associated with the shift of the sample specimen surface, as shown for the AgInS₂ sample. In our case, we did not observe any compounds of higher oxidation states of sulfur, such as sulfites or sulfates, and these correspond to the oxidized samples and Cu₂S, which could be formed due to the incomplete reaction of gallium. The absorption spectrum shows a red edge of approximately 600 nm.

A Tauc plot for as-synthesized CuGaS₂ NCs is shown in Figure 7F. The band gap E_bg = 2.56 eV was determined for these NCs. The Raman spectrum confirmed the formation of CuGaS₂ and showed good agreement with previously reported literature data (Zhong et al., 2011).

EDX analysis showed the composition of nanocrystals: Cu, 30.0%; Ga, 31.6%; and sulfur, 38.4 % wt (Figure 8). CuGaS₂ has the following composition: Cu, 32.19%; Ga, 35.32%; and S, 32.49%. The copper: gallium: sulfur ratio is 1:1:2.1. This confirms that excess sulfur used during the synthesis provides the sulfur-rich NCs. Distribution maps of chemical elements are identical for copper, gallium, and sulfur; therefore, this confirms CuGaS₂ as the single compound forming the nanocrystals.

Figure 8

Figure 8. CuGaS₂ NCs. (A) EDX spectrum; (B) TEM image; (C) sulfur distribution map; (D) Cu distribution map; (E) Ga distribution map.

XPS spectra were used to characterize the nanoparticle composition more accurately, as shown in Figure 9. Only copper, gallium, sulfur, carbon, oxygen, and nitrogen signals could be observed in this spectrum, indicating the contamination-free nature of the prepared nanocrystals (Figure 3). Carbon, nitrogen, and oxygen are present due to the organic ligand shell and adsorbed carbon dioxide from the atmosphere. The following elemental composition by weight was established from the XPS spectrum: Cu, 6.82%; Ga, 24.93%; and S, 68.26%. The copper: gallium: sulfur ratio is 1:3:20. These results additionally indicate that the nanoparticle surface is enriched in sulfur. Since the studied nanoparticles are several nanometers in size and possess a long-chain organic ligand shell with an approximate thickness of 1 nm, the elemental composition results differ from EDX. While EDX penetrates the material to depths of several micrometers, XPS analyzes only the upper ∼3 nm, thus primarily revealing the surface elemental composition of the nanoparticles.

Figure 9

Figure 9. XPS CuGaS₂ NCs. (A) XPS survey spectrum and high-resolution XPS scan spectra over the (B) Cu 2p region, (C) Ga 3p region, and (D) S 2p region.

Both copper 2p and gallium 3p signals could be perfectly fitted by the Gaussian (50%)–Lorentzian (50%) profile, indicating a single oxidation state for both metals. Gallium 3p_1/2 and 3p_3/2 peaks are observed at 109.7 eV and 106.2 eV, respectively. The Cu 2p spectrum is split into Cu 3p_1/2 (952.5 eV) and Cu 3p_3/2 (932.7 eV) (Figure 9B). Copper (I) as the only constituent could be assumed from the peak position (Kundu et al., 2008). The high-resolution sulfur spectrum exhibits a slightly broadened peak centered at 162.9 eV, resulting from the overlap of S 2p_3/2 and S 2p_1/2 signals. This fact allows us to exclude oxidation of our samples to sulfates or sulfites because these signals are normally observed at higher energies (167 eV–169 eV).

3.3 Ag₉GaS₆ NCs synthesis

The reaction of silver stearate, gallium stearate, and sulfur precursor requires temperatures as high as 190 °C for the formation of ternary NCs. Higher reaction temperature leads to the decomposition of the silver salt in this case. Application of a new precursor for the synthesis of AgGaS₂ under the reaction conditions analogous to those of CuGaS₂ (Haque et al., 2024) provided another unexpected ternary silver–gallium sulfide. Surprisingly, XRD analysis identified synthesized nanocrystals with the Ag₉GaS₆ phase (Brandt and Krämer, 1976). To the best of our knowledge, this compound was never obtained in the form of colloidal nanoparticles. Only a single report from 2001 describes hydrothermally prepared nanoparticles with the chemical structure of Ag₉GaS₆ having an average size of approximately 20 nm (Junqing et al., 2001). The obtained material was referred to as a “precipitate” or “powder,” and no data on colloidal stability or any evidence supporting a colloidal nature were provided. Thus, our study is the first report describing the preparation and characterization of Ag₉GaS₆ colloidal nanocrystals.

Ag₉GaS₆ could form two crystalline phases, and these XRD patterns are published (Lin et al., 2018; Hellstrom and Huggins, 1980). Alpha-Ag₉GaS₆ has an orthorhombic structure, and beta-Ag₉GaS₆ forms a cubic structure with space group F-43 m. The XRD pattern of the synthesized nanocrystals corresponds to beta-Ag₉GaS₆.

The formed nanoparticles are spherical with a mean size of approximately 9 nm (Figure 10). The formed nanoparticles possess a hydrophobic ligand shell. XRD-based size estimation using the Scherrer equation yields an average nanocrystallite diameter of 6.0 ± 0.7 nm, suggesting that a portion of the nanoparticle material may be amorphous.

Figure 10

Figure 10. Characterization of Ag₉GaS₆ NCs. (A) TEM image; (B) size-distribution diagram; (C) XRD-pattern; (D) absorption spectra in TCE; (E) Tauc plot; (F) Raman spectrum.

The prepared colloidal solutions of pure Ag₉GaS₆ were relatively unstable and agglomerate when stored for more than 48 h after isolation. The only report of monocrystalline Ag₉GaS₆ was the hydrothermal preparation of long whisker-like particles with an average size of 20 × 5 nm; these were never obtained in colloidal form (Junqing et al., 2001). The band gap of approximately 2.82 eV could be estimated from the Tauc plot of the prepared nanomaterial (Figure 10E). To the best of our knowledge, the Raman spectra of Ag₉GaS₆ have not yet been reported in the literature. Owing to its structural similarities with AgGaS₂ and CuGaS₂, the Raman spectrum of Ag₉GaS₆ exhibits comparable spectral features to those of these compounds, including analogous peak positions associated with metal–sulfur vibrations (Azhniuk et al., 2022). The highest-intensity peak was observed at 282 cm^-1, with two weaker satellites at 372 cm^-1 and 170 cm^-1 (Figure 10F).

EDX analysis confirms the presence of silver-rich nanoparticles (Figure 11), with an elemental composition by weight 63.7% Ag, 21.3% S, 15.0 Ga, consistent with Ag₉GaS₆, and indicates some excess gallium on the nanoparticle surfaces. The calculated chemical composition for Ag₉GaS₆ is Ag, 78.74; Ga, 5.66; and S, 15.60 and that for AgGaS₂ is Ag, 44.62; Ga, 28.84; and S, 26.53. Regular uniform distribution of Ag, Ga, and sulfur was confirmed by the EDX element distribution maps (Figure 11). Based on this analysis, the formation of Ag₂S NCs as a by-product could be excluded.

Figure 11

Figure 11. Characterization of Ag₉GaS₆ nanocrystals. (A) EDX spectrum; (B) TEM image; (C) Ag distribution map; (D) Ga distribution map; (E) S distribution map.

XPS spectra were used to characterize the nanoparticle composition more accurately, as shown in Figure 12. Only silver, gallium, sulfur, carbon, oxygen, and nitrogen signals could be observed in this spectrum, indicating the contamination-free nature of the prepared nanocrystals (Figure 12). Carbon, nitrogen, and oxygen are present due to the organic ligand shell and adsorbed carbon dioxide from the atmosphere. The following elemental composition by weight was established from the XPS spectrum: Ag, 38.0%; Ga, 2.9%; and S, 59.0%. The silver: gallium: sulfur ratio is 8.5:1:44, thus confirming Ag₉GaS₆ as a major chemical component of the nanoparticles. These results additionally indicate that the nanoparticle surface is enriched in sulfur. Since the studied nanoparticles are several nanometers in size and possess a long-chain organic ligand shell with an approximate thickness of 1 nm, the elemental composition results differ from those of EDX.

Figure 12

Figure 12. FT-IR spectrum of the synthesized colloidal NCs: (A) CuInS₂ and CuGaS₂ NCs; (B) AgInS₂ and Ag₉GaS₆ NCs; (C)1,800 cm^-1–1,100 cm^-1 region of the FT-IR spectra.

Both silver 3d peaks could be perfectly fitted by the Gaussian (50%)–Lorentzian (50%) profile, indicating a single oxidation state. Gallium 3p signals are overlapped with Si 2p from the silicon wafer. The Ag 3d spectrum is split into Ag 3d_3/2 (375.5 eV) and Ag 3d_5/2 (369.2 eV) (Figure 9B). Gallium 3p_1/2 and 3p_3/2 peaks could be estimated at 106.3 eV and 104.5 eV, respectively, by deconvolution (Figure 9C). The high-resolution sulfur spectrum exhibits a slightly broadened peak centered at 163.0 eV, resulting from the overlap of S 2p_3/2 and S 2p_1/2 signals. This allows us to exclude oxidation and the presence of sulfates or sulfites on the surface of our nanoparticles.

In summary, ternary chalcogenide nanoparticles with mean sizes ranging from 5 to 22 nm can be synthesized using a solution of elemental sulfur in decene-1, prepared at 170 °C, without any traces of binary chalcogenides as byproducts (Table 3). Gallium ternary sulfide NCs require higher temperatures of approximately 190 °C for preparation, whereas indium-based ternary sulfide NCs can be obtained within a temperature range of 150 °C–190 °C. The obtained morphologies of NCs are spherical for CuInS_2, CuGaS₂, and Ag₉GaS₆ and pear-shaped for AgInS₂ (Supplementary Figures S5–S8).

Table 3

Table 3. Synthesis of ternary nanocrystals in hexadecane with the sulfur-precursor synthesized at 170 °C.

3.4 FT-IR study of ABS₂ NCs

The FT-IR spectrum of the synthesized nanocrystals with a Cu(Ag):In(Ga):S precursor ratio of 1:1:100 revealed intensive broad absorption peaks in mid-IR for both copper indium sulfide and copper gallium sulfide (Figure 9). The maximum of the CuGaS₂ intra-band absorption peak was observed at 1,550 nm; for samples of CuInS₂, the maximum is red-shifted to 1,660 nm. The FWHM of these peaks for both samples is approximately 5500 nm. The highest intensity of the absorption peak is observed for CuInS₂ samples. These peaks could be attributed to the intra-band absorption. AgInS₂ and AgGaS₂ do not exhibit any intra-band absorption peaks in the mid-IR range of 2000 nm -10000 nm.

The ligand shell of colloidal NCs was analyzed by FT-IR. It should be noted that ternary ABX₂ chalcogenide NCs with carboxylate-type ligands are scarcely investigated. Nevertheless, carboxylate ligand shells are advantageous for thin-film preparations due to the easier exchange of carboxylates and mercaptans. Due to the two metals in NCs as possible binding sites for carboxylates and the different binding modes of carboxylates, the observed pattern is complex.

FT-IR spectra of gallium, indium, and silver stearates were recorded on the HATR-accessory for comparison (Supplementary Figures S9, S10). Asymmetrical vibrations of carboxylate were recorded at 1,552 cm^-1 for gallium and indium and at 1,517 cm^-1 for silver carboxylate.

The recorded spectra of NCs revealed that despite the similar composition of the reaction mixture and applied surfactants, the constitution of the synthesized nanocrystals’ ligand shell depends on the shape and composition of the semiconductor core. Since both stearate ions and oleylamine are present in the applied reagents, they could be found in the ligand shell in different ratios (Figure 13).

Figure 13

Figure 13. XPS Ag₉GaS₆ NCs. (A) XPS survey spectrum and high-resolution XPS scan spectra over the (B) Ag 3d region, (C) Ga 3p region, and (D) S 2p region.

Valence vibrations of CH bonds of long-chain aliphatic tails of stearate and oleylamine could be observed at 2,954 cm^-1, 2,925 cm^-1, and 2,854 cm^-1. Valence vibrations of cis-olefinic of the oleylamine moiety could be observed at 3,003 cm^-1. In addition, signals of the aliphatic chain belonging to the scissoring of methylene groups and rocking of the methyl group are detected at 1,467 cm^-1 and 1,376 cm^-1, respectively.

Studies of the carboxylate vibrations of long-chain carboxylates bound to NCs revealed complex patterns (Shukla et al., 2003). In our case of ABS₂, NC vibration of the carboxylate group could be observed at 1,590 and 1,514 cm^-1 for asymmetrical vibrations and 1,411 cm^-1 for symmetrical vibrations. Scissoring vibrations of the amino-group of oleylamine were observed at 1,634 cm^-1. Both indium-containing NCs possess stronger absorption bands at 1.517 ± 3 cm^-1 and 1.411 ± 2 cm^-1 compared to their gallium counterparts, indicating a higher affinity for carboxylate ligands.

From the FT-IR spectrum, it could be observed that a small part of stearate from the reagent applied exists in the protonated form bound to the NC surface (Cass et al., 2013). Free carbonyl vibrations could be found at 1,714 cm^-1 with an intensity below 1%.

4 Conclusion

In conclusion, the new reagent, obtained by the dissolution of elemental sulfur in decene-1, was developed and applied for the preparation of eco-friendly ternary sulfide nanocrystals ABS₂ (A = Cu and Ag; B = Ga and In). The impact of the reaction conditions on the chemical nature of this sulfur precursor was studied by several physico-chemical methods, including 2D NMR and GC–MS. This study revealed decyl disulfide as one of the possible active sulfur-transfer reagents in the synthesis of nanoparticles. The dependence of the morphological and optical properties of CuInS₂ NCs on the indium and sulfur precursors applied was studied in detail. The effect of hydrocarbon solvents with different chain lengths on nanocrystal synthesis was further analyzed, revealing that the more viscous hexadecane produced smaller nanoparticles with a narrower size distribution than decane.

The effectiveness of the new sulfur precursor was also demonstrated in the synthesis of CuGaS₂ and AgInS₂. In the case of the silver–gallium–sulfide system, the previously unknown Ag₉GaS₆ colloidal NCs were obtained for the first time by applying the new sulfur reagent. In general, the reagent yields spherical nanoparticles with an average size ranging from 5 to 20 nm; however, in the case of AgInS₂, unusual pear-shaped nanocrystals were formed. All obtained nanocrystals were thoughtfully characterized by XRD, TEM, EDX, FT-IR, and Raman spectroscopy. The FT-IR study revealed the mixed oleylamine/stearate nature of the ligand shell in the prepared nanocrystals, which is well-suited for further modification. The FT-IR analysis indicated that the ligand shell of the synthesized nanocrystals consists of oleylamine and stearate ligands, providing a versatile surface chemistry that is well-suited for subsequent functionalization or modification. The new reagent allows simple access to a range of eco-friendly ternary sulfide nanocrystals and, therefore, represents a versatile contribution to sustainable chemistry.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author contributions

IS: Conceptualization, Writing – review and editing, Supervision, Writing – original draft. VL: Investigation, Writing – review and editing, Data curation, Visualization. ArS: Investigation, Writing – review and editing, Visualization, Data curation. AeS: Visualization, Investigation, Writing – review and editing. OV: Investigation, Writing – review and editing, Visualization. GZ: Visualization, Investigation, Writing – review and editing. DV: Writing – review and editing. VI: Writing – review and editing, Funding acquisition.

Funding

The author(s) declared that financial support was received for this work and/or its publication. The research is supported by the Ministry of Science and Higher Education of the Russian Federation in the framework of Agreement No. 075-15-2025-608.

Acknowledgements

The authors thank A. Nursullaeva for GC–MS measurement. XPS studies were performed using the unique scientific equipment of the Kurchatov complex for synchrotron–neutron research of the National Research Center “Kurchatov Institute.”

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fnano.2025.1677927/full#supplementary-material

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