Formation and Structure of p-Nitrobenzoic Acid Adlayer on Au(111) Surface in HClO₄ Investigated by In-Situ Scanning Tunneling Microscopy

Abstract

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.

Revealing the interfacial water structure on a p-nitrobenzoic acid specifically adsorbed Au(111) surface

“…, 17–19 where it specifically adsorbs as p -nitrobenzoate (the PNBA − ion) on the Au(111) surface via the oxygen atoms of carboxylate. 20–22 Similar sharp bands at approximately 3630 cm −1 were observed for other carboxyl-containing adsorbates, such as glycine and fluoroacetate. 23,24 To understand the structural transformation of interfacial water accompanied by the sharp band appearing, three possible structures have been guessed: the formation of an in-plane confined water layer under the ordered adsorption of the PNBA − ions; some non-hydrogen-bonded water OH moieties and isolated water molecules embedded in the adlayer; or water molecules hydrogen-bonded to the adsorbed PNBA − ions.…”

“…The high-resolution scanning tunneling microscopy (STM) image of PNBA − adsorbed on the Au(111) surface, measured by the Wan group ( Fig. 1b ), 22 showed that the angle between a pair of the elongated blobs was approximately 120° in a (3 × 5) unit cell. To interpret the measured STM image, we compared it with the calculated EC-STM images of three possible Au(111)(3 × 5)–2PNBA − configurations (Fig.…”

Accurate Simulations of Scanning Tunneling Microscope: Both Tip and Substrate States Matter

“…Besides continuously improving the measured resolution, a ready alternative strategy is a combination of STM measurements with other surface-sensitive techniques, particularly the vibrational spectroscopies that contain molecular “fingerprints”. For example, an ordered domain of p (3 × 5) was found when p -nitrobenzoic acid (PNAB), which has been widely applied in fuel cells, catalysis, and photodetection, specifically adsorbed on the Au(111) electrodes under the applied electrode potential above 0.44 V vs the standard hydrogen electrode (SHE). , A typical electrochemical STM (EC-STM) image recorded at 0.66 V is depicted in Figure a, which indicates two PNBA molecules adsorbed in the p (3 × 5) supercell. , It is noted that we cannot directly infer any atomistic details from the image because of the blurry resolution. In fact, once the p (3 × 5)-2PNBA configuration was constructed, regardless of the protonation state for the adsorbed PNBA and the configuration for the coadsorbed water molecules, most of the calculated STM images can well reproduce the observed stripes (Figure a).…”

“…For example, an ordered domain of p(3 × 5) was found when p-nitrobenzoic acid (PNAB), which has been widely applied in fuel cells, catalysis, and photodetection, specifically adsorbed on the Au(111) electrodes under the applied electrode potential above 0.44 V vs the standard hydrogen electrode (SHE). 61,63 A typical electrochemical STM (EC-STM) image recorded at 0.66 V is depicted in Figure 6a, which indicates two PNBA molecules adsorbed in the p(3 × 5) supercell. 61,63 It is noted that we cannot directly infer any atomistic details from the image because of the blurry resolution.…”

“…61,63 A typical electrochemical STM (EC-STM) image recorded at 0.66 V is depicted in Figure 6a, which indicates two PNBA molecules adsorbed in the p(3 × 5) supercell. 61,63 It is noted that we cannot directly infer any atomistic details from the image because of the blurry resolution. In fact, once the p(3 × 5)-2PNBA configuration was constructed, regardless of the protonation state for the adsorbed PNBA and the configuration for the coadsorbed water molecules, most of the calculated STM images can well reproduce the observed stripes (Figure 6a).…”