Formation and structure of p-nitrobenzoic acid adlayer on Au(111) surface in HClO4 investigated by in-situ scanning tunneling microscopy.

The adsorption and adlayer structure of p-nitrobenzoic acid (pNBA) on a Au(111) surface in 0.1 M HCIO4 are investigated by in-situ scanning tunneling microscopy (STM) and cyclic voltammetry. The pNBA adlayer is prepared by dipping Au(111) into a saturated pNBA aqueous solution. The cyclic voltammogram (CV) of the so-prepared Au(111) electrode shows an irreversible cathodic peak at 0.24 V, which corresponds to the electro-reduction reaction from pNBA to hydroxylamine, and a pair of reversible peaks at ca. 0.7 V, corresponding to redox reaction of hydroxylamine to nitrosobenzoic acid. STM is employed to investigate the pNBA adlayer structure. The molecules form ordered adlayers in (16 x 2 square root of 3), (7 x 4 square root of 3) and (9 x 6 square root of 3) structures at the double layer potential region. High resolution STM image reveals the details of the molecular arrangement in the adlayers. On the basis of the STM image and the chemical structure of pNBA, structural models for three adlayers are proposed. In all three structures, pNBA molecules adsorb perpendicularly with the carboxylic groups contacting with the Au(111) substrate. The effect of dipping time on the adlayer formation is investigated. Upon shifting the electrode potentials in negative direction to induce the electroreduction reaction, the bright dots corresponding to the molecular aggregations start emerging on the surface and selectively locate at the elbow position along Au(111) reconstruction lines. The results provide direct evidence for the adsorption and electrochemical behavior of nitro-benzene derivatives.