Synthesis, Crystal Structure, Spectroscopic Properties, and Visible-Light Photoresponse of Lead-Free Cs₂PdCl₄·H₂O

Lead-free palladium halides containing square-planar coordination units provide a distinctive platform for investigating light-matter interactions, yet their spectroscopic behavior and thin-film photoresponse remain largely unexplored. Herein, we report a systematic investigation of Cs₂PdCl₄·H₂O, synthesized as both nanorods and single crystals, covering its structural, spectroscopic, electronic, and photoresponsive properties. Single-crystal X-ray diffraction confirms that the monohydrate crystallizes in the orthorhombic Cmcm space group with discrete square-planar [PdCl₄]^2- units. Powder X-ray diffraction verifies that the nanorods adopt the same crystalline phase and retain their principal diffraction features after two months of ambient storage. X-ray photoelectron spectroscopy confirms the Pd(II) valence state, and optical measurements determine a direct bandgap of 2.32 eV. Temperature-dependent and time-resolved photoluminescence reveal a low-temperature emission blueshift, enhanced emission intensity, and prolonged carrier decay, attributable to suppressed non-radiative relaxation and contributions from multiple emissive states. Density functional theory calculations based on the experimentally determined monohydrate structure show that the band-edge states are dominated by Cl 3p and Pd 4d orbitals. Thermally deposited Cs₂PdCl₄·H₂O-derived films deliver reproducible visible-light photocurrent switching across 397-564 nm and retain 71% of their initial response after 504 h under nitrogen. These findings establish Cs₂PdCl₄·H₂O as a visible-light-responsive lead-free palladium halide and provide a basis for future exploration in optoelectronic sensing.