Surface-Enhanced Vibrational and TPD Study of Nitroaniline Isomers

Abstract

para-nitroaniline isomers (ONA, MNA, and PNA, respectively) were investigated with surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman spectroscopy (SERS) via adsorption of the individual isomers on vacuum-deposited silver films and on silver powders of varying physical dimensions using temperature-programmed desorption (TPD). PNA exhibited a strong SERS signal, whereas the ONA and MNA isomers showed more moderate enhancement. Only the MNA isomer exhibited SEIRA. Varying the polarity of the deposition solvent changed the MNA adsorbate orientation and the degree of vibrational enhancement. Solvent experiments with MNA were the first to show SEIRA enhancement of the -NH 2 symmetric and antisymmetric stretch modes. TPD and vibrational experiments show evidence for nitroaniline intermolecular hydrogen bonding on 100-µm silver powder while suggesting stronger nitroaniline adsorption to 100-nm silver powder. Resonance effects, adsorbate geometries, and hydrogen-bonding variations combined with changes in the strength of adsorption explain the differences in SEIRA versus SERS enhancement of the individual isomers, solvent effects, and desorption characteristics.

Impact of choice of alkane deposition solvent on 4-fluorobenzoic acid layers adsorbed on silver nanostructures

“…1 SEIRA spectrum (bottom) of the 5 lL exposure of 4FBA adsorbed from n-pentane there is a band at 1397 cm À1 attributed to a COO stretching/ring deformation mode(s) from 4-fluorobenzoate ion (4FBI) formation in the monolayer. It is well known that benzoic acids ionize to benzoate ions at least in the monolayer during deposition on most transition metal surfaces [15][16][17][18][19][20]. The COO stretching band does not grow much in intensity as the amount of 4FBA deposited onto the silver film is increased in Fig.…”

“…Incorporation of n-heptane into organic films was seen to lesser degrees when ions of other halogenated benzoic acids were deposited onto silver films including 2-and 3-fluorobenzoic acids and all isomers of the chloro, bromo, and iodobenzoic acids. In other studies on silver nanostructures, uptake of n-heptane was never observed into films of isomers of nitrobenzoic acids, aminobenzoic acids, hydroxybenzoic acids, cyanobenzoic acids, aminophenols, aminobenzonitriles, or nitroanilines [15][16][17][18][19].…”

“…CaF 2 windows were polished with a Buehler Mastermet 2 colloidal silica suspension, rinsed and sonicated in nanopure water, and air dried before usage. It has been previously demonstrated that a seven nanometer silver film as determined by the QCM is optimal for SEIRA [16]. Earlier scanning electron and atomic force microscopy studies have shown these silver films to be rough with an average particle size of 50-100 nm in diameter and an average particle height of about 5 nm [16,19].…”

“…It has been previously demonstrated that a seven nanometer silver film as determined by the QCM is optimal for SEIRA [16]. Earlier scanning electron and atomic force microscopy studies have shown these silver films to be rough with an average particle size of 50-100 nm in diameter and an average particle height of about 5 nm [16,19]. Henceforth, the phrase a 'silver film' is with reference to seven nanometers of silver as determined by the QCM evaporated on a CaF 2 substrate.…”

“…On the other hand, the more famous surface-enhanced Raman spectroscopy (SERS), being a more short-range phenomenon, cannot be applied routinely to studies in the multilayer. Along these lines, it has been shown that SEIRA can enhance the infrared signal of an adsorbate on rough silver films from the sub-monolayer all the way through a multilayer coverage [15][16][17][18][19].…”