Decoding Vibrational Signatures of Molybdenum Sulphide Molecular Catalysts for Solar Hydrogen Evolution

Abstract: Molybdenum sulfide clusters, [Mo3S4]4+ and [Mo3S13]2-, have emerged as key molecular models for understanding active sites in Mo-S-based catalysts and as promising candidates for energy conversion applications. Despite their importance, comprehensive vibrational characterization of these clusters remains limited. Here, we present a detailed Raman and infrared spectroscopic analysis of both clusters, supported by density functional theory (DFT) calculations. High-quality crystalline samples were synthesized and characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to confirm morphology and stoichiometry. Raman spectra, acquired using 488 nm and 532 nm laser excitation, were deconvoluted using Lorentzian fitting. Vibrational mode assignments were made through direct comparison with DFT predictions. For [Mo3S4]4+, major Raman bands appear near 200 cm^-1, 350 cm^-1, and 450 cm^-1, corresponding to Mo-S-Mo bending, Mo-S stretching, and terminal sulfur vibrations. [Mo3S13]2- shows two distinct spectral regions: 100-400 cm^-1 for Mo-S and S-S bending and stretching, and 450-550 cm^-1 for terminal disulfide (S-S) stretching. Complementary IR spectra calculations reveal additional vibrational features, yielding a more complete fingerprint for each cluster. Finally, we demonstrate that Raman spectroscopy offers greater sensitivity than X-ray diffraction (XRD) in detecting these clusters on supporting materials. This work provides a detailed vibrational reference for [Mo3S4]4+ and [Mo3S13]2-, establishing Raman and IR spectroscopy as powerful tools for characterizing Mo-S molecular clusters in both fundamental and applied contexts.