Probing NO₂ Reactivity on Coinage Metal Surfaces through Liquid Crystal Orientational Responses

Understanding the reactivity of nitrogen oxides (NO _x ) at interfaces remains a challenge for environmental monitoring and air quality control. Here, we show that 4-cyano-4'-pentylbiphenyl (5CB) liquid crystals (LCs) supported on coinage metal Au, Ag, and Cu surfaces exhibit NO₂-induced orientational changes that provide an optical probe of NO₂ adsorption and interfacial interactions on metal surfaces at parts-per-million (ppm) concentrations. Films of Au were prepared by electron-beam deposition and subsequently coated with submonolayer to multilayer films of Ag or Cu via electrochemical deposition. X-ray photoelectron spectroscopy reveals that these Ag and Cu films oxidize under ambient conditions to form oxygen-decorated Ag and Cu₂O surface oxides, respectively. By combining polarized light microscopy, polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and density functional theory (DFT) calculations, we show that 5CB adopts a planar orientation on Au and Cu overlayers on Au (Cu/Au) surfaces, but a perpendicular alignment on Ag overlayers on Au (Ag/Au). Upon exposure to 10 ppm of NO₂ balanced in N₂ at ambient pressure and temperature, 5CB undergoes substrate-dependent orientational transitions. On Au, a reversible planar-to-perpendicular transition is observed, which DFT calculations attribute to electrostatic stabilization of the perpendicular binding mode of 5CB by adsorbed NO₂. On Cu/Au, the same transition in orientation of the LC occurs, but it is irreversible and correlates with the oxidation of Cu₂O to CuO by NO₂. Over ambient-exposed Ag/Au films, however, 5CB is unresponsive to NO₂, retaining a perpendicular alignment before and after exposure. DFT calculations indicate that the presence of the LC modifies surface reaction thermodynamics on Ag/Au, shifting the preferred NO _x species formed from N₂O₄ in the absence of LCs to NO₂ in their presence, and that NO₂ adsorption does not alter the preferred orientation of 5CB. Taken together, these results demonstrate how substrate-dependent surface composition and oxidation state govern NO₂ adsorption and speciation on coinage metal surfaces with and without LC overlayers.