Assessing quality and purity of MoS₂ nanosheets by diffuse reflectance IR spectroscopy

Liquid-phase processing is increasingly used to produce thin films and composites from nanomaterials, including 2D nanosheets derived from van der Waals crystals. However, an inherent problem is the exposure of the nanomaterial to organic compounds from the environment – solvents, intercalants, and stabilizers – which can adsorb and potentially degrade device performance, especially at electrode interfaces. For conventional washing methods it is often challenging to confirm complete removal, as these residues are difficult to detect spectroscopically. Here, we use Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy to analyse freeze-dried 2D MoS₂ nanosheets produced via various exfoliation methods. First-principles calculations predict infrared (IR) spectral shifts corresponding to nanosheet thickness in the few-layer regime (<10 layers), which were confirmed experimentally. We demonstrate that centrifugation effectively removes small-molecule surfactants (e.g., 2 g·L -1 sodium cholate or sodium dodecyl sulphate) used in liquid-phase exfoliation (LPE), while residues persist in MoS₂ sonicated in NMP or electrochemically exfoliated samples. This work provides a protocol to improve washing or monitor chemical modifications and highlights that IR-active vibrational modes, accurately predicted from first principles, can serve as a tool to determine 2D material layer numbers.