Self-Assembly of Anthraquinone-2-carboxylic Acid on Silver: Fourier Transform Infrared Spectroscopy, Ellipsometry, Quartz Crystal Microbalance, and Atomic Force Microscopy Study

Abstract

The adsorption of anthraquinone-2-carboxylic acid (AQ-2-COOH) in ethanol on a silver surface has been investigated by reflection−absorption infrared (RAIR) spectroscopy, ellipsometry, quartz crystal microbalance (QCM), and atomic force microscopy (AFM). The RAIR spectral data were found to be consistent with those gathered by other experimental means. Specifically, from the RAIR spectral data it was concluded that the molecule chemisorbs on silver as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the metal substrate. The molecular plane was determined to be tilted by 40° from the surface normal. The ellipsometric thickness of the monolayer, 10.6 ± 1.2 Å, agreed well with that predicted from the RAIR data, 10.4 Å. QCM data suggested that the self-assembling process could be described in terms of the Langmuir adsorption model, providing the value of the free energy of adsorption at −21.8 kJ/mol. The limiting surface coverage was determined from the QCM data to be 2.5 × 10-10 mol/cm2, corresponding to the area per adsorbate to be 65 Å2. The latter value was quite close to that predicted from the RAIR spectral data, 66 Å2/molecule. Neither a periodic nor a molecularly resolved image could be obtained under a contact AFM measurement, but the lateral force microscopy (LFM) images revealed that the anthraquinone moieties were close-packed on the silver surface, with the rings assembled parallel to one another to form a brick like architecture. The area per adsorbate estimated from the LFM images, 62 ± 5 Å2, was also consistent with that estimated from the QCM and RAIR data.