Characterizing Nanostructured Films Using Phase Sensitive Vibrational Sum Frequency Spectroscopy

Nanoparticle films are ubiquitous thermal and electrocatalysts, yet their operando characterization remainschallenging. Vibrational sum-frequency generation (vSFG) spectroscopy offers unique advantages due to itshigh sensitivity and surface specificity, but its application to systems with such intermediate length scaledisorder, particularly with phase-resolved detection, has been challenging. In this study, we describe anapproach to phase-resolved vSFG spectroscopy of nanoparticle films using z-cut α-quartz as a local reference.We show, by analysis of an octadecyltrichlorosilane (OTS) film on quartz under the ppp polarization condition,quantitative detection of absolute phase is possible and subsequently apply this protocol to a film of Mn-dopedCo₃O₄ nanoparticles. Two OH species are resolved (∼ 3585 and ∼ 3770 cm⁻¹), both oriented H-up relative tothe surface. This approach delivers a practical, internally referenced, phase-resolved vSFG methodology fornanoparticle ensembles on dielectric supports, and thus offers operando access to catalytic interfaces beyondmetallic or plasmonically enhanced systems.