Characterizing Nanostructured Films Using Phase Sensitive Vibrational Sum Frequency Spectroscopy

Nanoparticle films are ubiquitous thermal and electrocatalysts, yet their \textit{operando} characterization remains challenging. Vibrational sum-frequency generation (vSFG) spectroscopy offers unique advantages due to its high sensitivity and surface specificity, but its application to systems with such intermediate length scale disorder, particularly with phase-resolved detection, has been challenging. In this study, we describe an approach to phase-resolved vSFG spectroscopy of nanoparticle films using z-cut $\alpha$-quartz as a local reference. We show, by analysis of an octadecyltrichlorosilane (OTS) film on quartz under the \textit{ppp} polarization condition, quantitative detection of absolute phase is possible and subsequently apply this protocol to a film of Mn-doped \ch{Co3O4} nanoparticles. Two OH species are resolved ($\sim 3585$ and $\sim 3770~\text{cm}^{-1}$), both oriented H-up relative to the surface. This approach delivers a practical, internally referenced, phase-resolved vSFG methodology for nanoparticle ensembles on dielectric supports, and thus offers \textit{operando} access to catalytic interfaces beyond metallic or plasmonically enhanced systems.