One-Pot Synthesis of Binary and Ternary Metal Sulfide Nanocrystals via Metal‑Thiolate Decomposition

Abstract

We present a unified one‑pot route that yields high‑quality binary and ternary metal sulfide nanocrystals (NCs), including PbS, Cu₂S, ZnS, CdS, Ag₂S, CuInS₂, and Cu‑doped CdS. The method relies solely on inorganic metal salts and n-dodecanethiol (DDT), eliminating the need for pre‑formed organometallic precursors. A layered metal‑thiolate intermediate forms at the outset, which subsequently decomposes to nucleate NCs that grow with reaction time. The resulting CdS NCs exhibit a broad, weak surface‑state emission, while Cu(I) doping shifts the emission to the red, demonstrating tunable optical properties. This scalable approach can be extended to more complex multinary sulfide systems.

Background

Colloidal inorganic nanocrystals have garnered significant interest due to their unique size‑dependent optical and electronic behavior, enabling applications in light‑emitting diodes, bio‑labeling, photovoltaics, and memory devices. Metal sulfide NCs are especially attractive because their optoelectronic characteristics can be finely tuned by controlling size, shape, phase, and composition. Traditional synthesis strategies—hydrothermal, hot‑injection, and single‑source precursor methods—often involve cumbersome steps, limited scalability, or require air‑free handling. Moreover, the formation of doped or ternary sulfides via a simple, versatile route remains underexplored. Our group has previously demonstrated that inorganic salts combined with alkyl thiols can generate high‑quality metal sulfides. Building on that work, we introduce a straightforward, low‑cost, and general wet‑chemical approach that obviates toxic organometallic precursors and phosphine ligands.

Methods

Synthesis of Binary Metal Sulfide NCs

For PbS, 3 mmol of Pb(OAc)₂·3H₂O was mixed with 20 mL DDT in a three‑neck flask at room temperature. After nitrogen degassing (≈20 min), the mixture was heated to 200 °C and held for 60 min. The reaction was quenched by cooling to ambient temperature. NCs were isolated by adding ethanol, centrifuging at 7000 rpm for 10 min, and washing twice with chloroform. The purified NCs were either dispersed in chloroform or dried under vacuum for characterization.

Cu₂S NCs were prepared by combining 3 mmol Cu(acac)₂ with 10 mL DDT and 20 mL ODE, then heating to 200 °C for 60 min. ZnS NCs required 3 mmol Zn(acac)₂, 5 mL DDT, 25 mL ODE, and 240 °C for 180 min. CdS NCs were obtained from 5 mmol Cd(acac)₂ and 30 mL DDT, heated to 200 °C for 23 h. Ag₂S NCs followed a similar protocol to PbS but with 205 °C for 100 min.

Synthesis of Ternary Metal Sulfide NCs

CdS:Cu(I) NCs were synthesized by mixing 4.5 mmol Cd(acac)₂ and 0.5 mmol Cu(acac)₂ in 30 mL DDT, then heating to 200 °C for 23 h. CuInS₂ NCs were prepared with 3.1 mmol Cu(acac)₂, 1.9 mmol In(acac)₃, 5 mL DDT, and 25 mL ODE, heated to 240 °C for 60 min. Detailed experimental conditions are summarized in Table 1.

Characterization

Transmission electron microscopy (TEM, Hitachi‑7650, 100 kV) and high‑resolution TEM (HRTEM, JEM‑2010, 200 kV) revealed NC morphology and lattice fringes. X‑ray diffraction (XRD, Bruker D8 Advance, Cu Kα) confirmed crystal phases. X‑ray photoelectron spectroscopy (XPS, VG Scientific ESCALab220i‑XL, Al Kα) provided compositional and valence‑state information, calibrated against the C 1s line at 284.8 eV. UV‑Vis absorption (Shimadzu‑UV 3101) and photoluminescence (Varian Cary Eclipse) measured optical properties in chloroform solutions.

Results and Discussion

The synthesis proceeds through the formation of a layered metal‑thiolate intermediate, which decomposes to nucleate NCs. Figures 1 and 2 illustrate the reaction scheme and the characteristic gel‑to‑liquid transformation observed at early stages. XRD patterns of the intermediates (Fig. 3) show sharp (0k0) reflections indicative of a double‑layer DDT and metal ion arrangement, confirming the polymeric nature of the intermediate.

Phase analysis of the final products (Fig. 4) matched standard patterns: PbS (fcc, JCPDS 77‑0422), Cu₂S (hexagonal, JCPDS 26‑1116), ZnS (cubic zinc blende, JCPDS 80‑0020), CdS/CdS:Cu(I) (cubic, JCPDS 10‑0454), Ag₂S (monoclinic, JCPDS 14‑0072), and CuInS₂ (wurtzite). The incorporation of Cu(I) into CdS slightly perturbs the lattice, as evidenced by minimal shifts in XRD peaks.

Cu(I) valence in CdS:Cu(I) NCs was confirmed by XPS: the Cu 2p spectrum shows peaks at 952 eV and 932.4 eV with no shake‑up feature, consistent with a +1 oxidation state. TEM images (Fig. 6) reveal that Cu₂S NCs are ~8 nm spheres with hexagonal close‑packed ordering, PbS NCs are ~94 nm octahedra, while CdS, ZnS, Ag₂S, and CuInS₂ NCs are quasi‑spherical or bullet‑shaped with sizes below 10 nm. HRTEM confirms lattice spacings corresponding to their respective crystal planes.

Optical studies (Fig. 7) demonstrate that undoped CdS emits green fluorescence (~548 nm, PL QY ≈ 10 %) with a broad surface‑state band, whereas Cu(I) doping shifts emission to 642 nm and increases PL QY to ~16 %. Absorption maxima also red‑shift from 364 nm (CdS) to 384 nm (CdS:Cu(I)), independent of size, confirming dopant‑induced band‑tail states. Varying the Cu/Cd precursor ratio (7:3, 9:1, 19:1) further tunes the absorption and emission wavelengths, with higher Cu content yielding dominant 710 nm emission from deep donor‑acceptor recombination.

Conclusions

We have established a simple, scalable one‑pot strategy that produces a broad family of binary and ternary metal sulfide NCs without pre‑forming organometallic precursors. Layered metal‑thiolate intermediates enable precise control over size, shape, and composition. The method delivers high‑quality, red‑emissive CdS:Cu(I) NCs, demonstrating its potential for doped NCs in optoelectronic devices. The versatility of this approach paves the way for synthesizing more complex multinary sulfides for photovoltaic and light‑emitting applications.

Abbreviations

DDT

n-dodecanethiol

HRTEM

High‑resolution transmission electron microscope

NCs

Nanocrystals

ODE

1‑octadecene

PL QY

Photoluminescence quantum yield

TEM

Transmission electron microscope

XPS

X‑ray photoelectron spectrometer

XRD

X‑ray diffractometer