Solvent-Directed Femtosecond Laser Ablation: Tuning Phase and Defect Engineering in Hybrid CdPS3/CdS Nanostructures

The limited visible-light absorption of wide-bandgap van der Waals crystals fundamentally restricts their utility in solar energy conversion. Here, we report a surfactant-free, solvent-directed laser synthesis strategy to engineer the phase and optoelectronic properties of Cadmium Phosphorus Trisulfide (CdPS3). By exploiting the non-equilibrium thermodynamics of femtosecond pulsed laser ablation in liquid (fs-PLAL), we demonstrate a tunable transition from the stoichiometric ternary phase to a highly active binary-rich heterostructure. While ablation in water preserves the monoclinic CdPS3 lattice, the reducing environment of isopropanol triggers the formation of CdS quantum dots and metallic cadmium defect sites. This solvent-induced phase engineering transforms the ultraviolet-active host into a robust visible-light photocatalyst. The resulting hybrid CdPS3/CdS nanocolloids exhibit superior charge separation efficiency, driven by Schottky-like metal-semiconductor junctions, achieving ~ 90% degradation of Methylene Blue under 532 nm irradiation within 30 minutes. This work establishes fs-PLAL as a scalable defect-engineering tool for complex ternary layered materials, offering a new design of high-performance metal-thiophosphate-based photocatalysts.

💡 Research Summary

This paper presents a novel, surfactant-free synthesis strategy to overcome the inherent limitation of wide-bandgap van der Waals semiconductors—specifically Cadmium Phosphorus Trisulfide (CdPS3)—in visible-light photocatalysis. The core methodology employs femtosecond Pulsed Laser Ablation in Liquid (fs-PLAL), where a bulk CdPS3 crystal is ablated within different liquid media. The groundbreaking finding is that the chemical nature of the solvent acts as a master switch, dictating the non-equilibrium thermodynamic pathway and ultimately engineering the phase, composition, and defect structure of the resulting nanocolloids.

When ablation is performed in deionized water, the high heat capacity and oxidative environment preserve the monoclinic crystal structure and ternary stoichiometry of CdPS3. In stark contrast, ablation in organic solvents, particularly the reducing environment of isopropanol (IPA), triggers a profound phase transformation. The extreme conditions within the laser-induced plasma plume, coupled with solvent interactions, promote the dissociation of CdPS3 and the preferential formation of the binary phase, cadmium sulfide (CdS), which has a lower formation energy. Comprehensive characterization using TEM, SAED, EDX, and Raman spectroscopy confirms this solvent-directed phase engineering. The IPA-synthesized colloid consists of nearly 90% CdS phase coexisting with residual CdPS3, forming a hybrid nanostructure.

Photoluminescence (PL) spectroscopy reveals the electronic consequences of this transformation. Under supra-bandgap excitation (390 nm), the water-synthesized sample shows a single PL peak (~435 nm) attributed to defect states in CdPS3. The acetonitrile (ACN) sample displays an additional peak at ~490 nm, signifying the presence of CdS quantum dots (QDs), evidenced by a blue shift from the bulk CdS bandgap due to quantum confinement. Intriguingly, the IPA sample, with the highest CdS content, exhibits the weakest PL intensity. This is interpreted as evidence for the formation of metallic cadmium (Cd0) defects under the strong reducing conditions. These metallic sites create Schottky-like junctions with the semiconductor components (CdS/CdPS3), acting as efficient electron sinks that quench radiative recombination—a process detrimental to PL but highly beneficial for photocatalysis by prolonging charge carrier lifetime.

The photocatalytic efficacy is rigorously evaluated through the degradation of Methylene Blue (MB) dye under 532 nm visible light irradiation, monitored via in-situ Raman spectroscopy. The hybrid CdPS3/CdS nanocolloids synthesized in IPA demonstrate superior performance, achieving approximately 90% degradation of MB within 30 minutes. In contrast, the water-synthesized CdPS3 nanoparticles show negligible activity. The performance enhancement is attributed to a synergistic mechanism: 1) extended visible-light absorption via the narrower bandgap CdS, and 2) drastically improved charge separation efficiency driven by the metal-semiconductor junctions formed by the metallic Cd0 defects.

In conclusion, this work establishes fs-PLAL as a powerful and scalable tool for defect and phase engineering in complex ternary layered materials. By simply selecting the solvent environment, one can precisely tailor material properties, moving from a UV-active semiconductor to a robust visible-light photocatalyst. This research opens a new, ligand-free design pathway for high-performance metal-thiophosphate-based hybrid photocatalysts for solar energy conversion and environmental remediation.