An Ordered Retinoate Monolayer Prepared on Rutile TiO2(110)

Abstract

An all-trans retinoate ion monolayer was prepared by a substitution reaction of all-trans retinoic acid with trimethyl acetate ion adsorbed on rutile TiO2 (110) in solutions. The prepared monolayer was assessed by infrared-visible sum-frequency generation (SFG) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). Two strong vibrational SFG bands were observed at 1580 and 1405 cm-1. The strong intensities of the bands were due to electronic resonance of the SFG signal. The 1580 cm-1 band was assigned to the predominantly in-phase stretching vibration of the conjugated CC double bonds of retinyl chromophore, and the 1405 cm-1 band to the symmetric −COO stretching vibration of carboxylate ions attached to TiO2 (110) with the two oxygen atoms on bridge-bonded 5-fold coordinated Ti atoms. The assignments suggest that retinoic acid is dissociatively adsorbed with its −COO group pointing to the substrate. Precovering TiO2 (110) substrate with trimethyl acetate was found to be crucial to the high quality of the monolayer based on control experiments. STM observation showed that no lateral order of retinoate existed on the monolayer.