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.