Hybrid CdPS₃/CdS Nanocolloids for Photocatalysis

• The paper demonstrates fs-PLAL synthesis that enables solvent-driven phase transformation and controlled defect engineering in hybrid CdPS₃/CdS nanocolloids.
• Structural characterization using TEM, SAED, and Raman spectroscopy reveals multiphasic nanostructures with CdS quantum dots and metallic Cd⁰ inclusions.
• Optoelectronic and photocatalytic studies show extended visible absorption and >90% efficiency in degrading organic dyes under green light irradiation.

Hybrid CdPS₃/CdS nanocolloids are engineered, multiphasic nanostructures derived from layered cadmium phosphorus trisulfide (CdPS₃) in combination with cadmium sulfide (CdS), often including controlled metallic cadmium (Cd⁰) defect sites. These hybrid systems are synthesized by femtosecond pulsed-laser ablation in liquid (fs-PLAL), which utilizes nonthermal plasma processes under solvent-dependent conditions to achieve tunable phase composition, defect density, and hierarchical architectures. This resulting complex exhibits enhanced optoelectronic and photocatalytic properties, notably broadening optical absorption from ultraviolet into the visible range and achieving high quantum-efficiency in redox reactions such as organic dye degradation (Ushkov et al., 9 Dec 2025).

1. Synthesis Methodologies: Femtosecond Pulsed-Laser Ablation in Liquid

Hybrid CdPS₃/CdS nanocolloids are synthesized using a fs-PLAL strategy, in which ultrashort laser pulses (Yb:KGW, λ₀ = 1030 nm, τₚ ≈ 400 fs, 200 kHz) are focused onto a layered CdPS₃ target immersed in a selected solvent. 10 minutes (1.2 × 10⁸ pulses) irradiation over a 2 × 2 mm region (scanned at 3 m/s) with a 50 µm spot size, generates transient high-temperature (∼10⁴ K), high-pressure plasma bubbles at the solid-liquid interface. The properties of the solvent (deionized water, isopropanol, or acetonitrile) act as a “phase master-switch,” modulating cooling rates and chemical reactivity within the plasma and determining the final nanocolloid composition. Water (high thermal conductivity, oxidizing) favors preservation of stoichiometric CdPS₃, while isopropanol (reducing, radical precursor) induces fragmentation and recombination into CdS quantum dots (QDs) and metallic Cd⁰ inclusions (Ushkov et al., 9 Dec 2025).

2. Structural and Compositional Characterization

Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) reveal broad particle size distributions (∼10–80 nm, mean ≈ 40 nm). Synthetic conditions dictate the crystal phases present:

• Water ablation produces monoclinic CdPS₃ nanocolloids (C2/m), as confirmed by matching SAED patterns.
• Isopropanol ablation yields biphasic nanocolloids: SAED shows superposed patterns from monoclinic CdPS₃ and wurtzite CdS (a = 3.82 Å, c = 6.25 Å), with blue-shifted features indicating QD formation (< exciton Bohr diameter, ≤ 6 nm).

Energy-dispersive X-ray (EDX) analysis quantifies solvent-dependent stoichiometry:

Solvent	Cd (at%)	P (at%)	S (at%)	CdPS₃ / CdS (mol %)
DI water	20.9	16.5	62.5	88 / 12
ACN	28.0	14.8	57.3	53 / 47
IPA	43.1	6.4	50.5	11 / 89

Raman spectroscopy further distinguishes phase composition. Water-synthesized colloids retain sharp CdPS₃ peaks (248, 272, 377 cm⁻¹). IPA-colloids exhibit broadened/diminished CdPS₃ signatures, new bands at ∼300 (CdS LO) and ∼600 cm⁻¹ (2LO), and phonon confinement/lattice strain effects consistent with rapid quenching (Ushkov et al., 9 Dec 2025).

3. Optical and Optoelectronic Properties

UV–Vis absorption spectra reveal a pronounced solvent effect. Water colloids absorb at ≈410 nm (band edge ≈3.02 eV), while IPA colloids extend absorbance to ≈490 nm (2.53 eV), in agreement with Tauc direct-gap analysis:

$(\alpha h\nu)^2 = A(h\nu - E_g)$

yielding $E_g$(water) ≈ 3.0 eV, $E_g$(IPA) ≈ 2.53 eV.

Photoluminescence (PL) is excitation-wavelength and phase dependent:

• $\lambda_{ex}$ = 390 nm: Water colloids emit at 435 nm (shallow CdPS₃ defect states, 0.14 eV below CB). IPA colloids show 435 nm (CdPS₃) and a new 490 nm emission (CdS QD exciton recombination, 2.53 eV).
• $\lambda_{ex}$ = 440 nm: Only CdS (IPA-derived) shows strong emission at ∼490 nm; water sample is inactive in this region.
• PL quenching is pronounced in IPA-derived nanocolloids compared to acetonitrile, attributable to Cd⁰ domains serving as Schottky electron sinks, facilitating nonradiative decay and suppressing recombination.

The combination of spectral features and strong PL quenching (without explicit lifetime measurements) implies charge-extraction efficiency surpassing 90%, with photogenerated electrons predominantly transferred to metallic Cd⁰ (Ushkov et al., 9 Dec 2025).

4. Photocatalytic Activity and Reaction Kinetics

CdPS₃/CdS nanocolloids synthesized in IPA exhibit high photocatalytic performance in organic dye degradation. In 1 µM methylene blue (MB) solutions (1:1 colloid:solvent), irradiated at 532 nm, IPA nanocolloids enable ≈90% MB degradation in 30 min:

$C_t = C_0 e^{-kt}$

$$k_{\mathrm{IPA}} \approx -(1/30\,\mathrm{min}) \ln(0.10) \approx 0.077\,\mathrm{min}^{-1}$$

Raman confocal monitoring (MB band at 1625 cm⁻¹) confirms the rapid disappearance of MB. Water-based colloids demonstrate negligible activity (Cₜ/C₀ ≈ 1), in accordance with their UV absorption profile (Ushkov et al., 9 Dec 2025).

Metallic Cd⁰ inclusions at the semiconductor interface play a pivotal role, functioning as electron-acceptor Schottky domains that extend the lifetime of hot electrons, potentiate interfacial charge transfer, and enhance overall quantum efficiency in visible-light photocatalysis.

5. Mechanistic Insights: Solvent-Driven Phase and Defect Engineering

The underlying mechanisms derive from extreme non-equilibrium dynamics during fs-PLAL:

• Laser energy deposition forms a transient, nanosecond-scale plasma bubble in the solvent.
• In water, rapid thermal dissipation preserves bulk stoichiometry and minimizes defect formation.
• In IPA, secondary-alcohol radicals scavenge phosphorus, promote Cd–S bond formation, and facilitate reduction of Cd²⁺ to Cd⁰, generating binary CdS QDs and metallic clusters.
• The resulting hybrid exhibits a type-II heterojunction: wide-bandgap CdPS₃ (E_g ≈ 3.0 eV) couples to narrower-gap CdS QDs (E_g ≈ 2.53 eV), extending photoresponse to the visible. Metallic Cd⁰ clusters form Schottky barriers with the CdS conduction band, acting as electron drains and minimizing electron–hole recombination (Ushkov et al., 9 Dec 2025).

6. Significance and Implications for Photocatalysis

The fs-PLAL method constitutes a surfactant-free, scalable approach for defect and phase engineering in complex ternary layered chalcogenides. The solvent-dependent strategy permits rational control over photonic, electronic, and catalytic functions. CdPS₃/CdS nanocolloids, especially those with metallic Cd⁰ inclusions, demonstrate robust, visible-light-activated photocatalytic removal of organic dyes, specifically achieving ~90% degradation of methylene blue within 30 min under green irradiation (532 nm). This suggests possible extensions of the technique to a wider class of metal-thiophosphate materials for solar-driven redox applications (Ushkov et al., 9 Dec 2025).