Aicheng Chen

Ionic adsorption at the Au(111) electrode

Abstract This paper reviews thermodynamic, spectroscopic and X-ray diffraction studies of SO 4 2− , Cl − , Br − and I − adsorption at the Au(111) electrode surface. The thermodynamic experiments provided information about surface concentrations of specifically adsorbed anions, Gibbs energies of adsorption and the numbers of unit charge flowing to the interface per one adsorbed anion. Second harmonic generation and surface X-ray scattering revealed the affect of ionic adsorption on the electronic and crystallographic structure of the interface. The atomistic information derived from these microscopic studies was used to discuss the thermodynamic data and to verify the theoretical models of the interface. It has been shown that complete description of ionic adsorption at metal electrodes requires a concerted use of the macroscopic and microscopic techniques.

Chloride adsorption at the Au(111) electrode surface

The adsorption of iodide at a Au(111) single crystal electrode has been investigated quantitatively using chronocoulometry. By analyzing the charge density data thermodynamically, the following parameters were determined: the Gibbs excess, Gibbs energy of adsorption, the number of electrons flowing to the interface per one adsorbed iodide ion at a constant electrode potential (electrosorption valency), and at a constant chemical potential. The thermodynamic data for iodide adsorption were compared to the results for bromide and chloride adsorption. All the three halides form a chemisorption bond with the gold surface. The bond is quite polar at the negatively charged surface, however, its polarity drops significantly at the positively charged surface. At low charge densities and coverages, the bond polarity is determined by the ability of free electrons to screen the dipole formed by the adsorbed anion and its image charge in the metal. At high charge densities and coverages, the chemisorption bond has a predominantly covalent character. The strength of the halide adsorption and the covalent character of the chemisorption bond increase progressively by moving from chloride to iodide.

A SNIFTIRS study of the adsorption of pyridine at the Au(111) electrode-solution interface

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been applied to study coordination of the pyridine molecules to the Au(111) electrode surface. The IR spectra have been recorded using both p- and s-polarized radiation. The ratio of the integrated band intensities for the spectra recorded with p- and s-polarized light was then used to study changes in the surface coordination of pyridine molecules. We have derived an expression describing the dependence of this ratio on the tilt angle. We have described the orientation of the adsorbed molecule in terms of angles formed between the surface, and: (i) C2v axis of the pyridine molecule, (ii) the direction in plane of the molecule and normal to the C2v axis. We were able to demonstrate that both angles increase by moving from negative to positive potentials. This result indicates that the pyridine molecule not only stands up at positive potentials but also its plane rotates somewhat with respect to the electrode surface.

Electrochemical and FTIR studies of L-phenylalanine adsorption at the Au(111) electrode

Abstract The adsorption of l -phenylalanine (Phe) at the Au(111) electrode surface has been studied using electrochemical techniques and subtractively normalized interfacial Fourier transform infrared (SNIFTIR) techniques. The electrochemical measurements of cyclic voltammetry, differential capacity and chronocoulometry were used to determine Gibbs energies of adsorption and the reference (E1) and sample (E2) potentials to be used in the spectroscopic measurements. The vibrational spectra have been used to determine: (i) the orientation of the molecule at the surface as a function of potential; (ii) the dependence of the band intensity on the surface coverage; (iii) the character of surface coordination, and (iv) the oxidation of adsorbed Phe molecules at positive potentials. The adsorption of Phe is characterized by ΔG values ranging from −18 to −37 kJ mol−1 that are characteristic for a weak chemisorption of small aromatic molecules. The electrochemical and SNIFTIR measurements indicated that adsorbed Phe molecules change orientation as a function of applied potential. At the negatively charged surface Phe is predominantly adsorbed in the neutral form of the amino acid. At potentials positive to the pzc, adsorption occurs predominantly in the zwitterionic form with the COO− group directed towards the surface and the ammonium group towards the solution. At more positive potentials electrocatalytic oxidation of Phe occurs and is marked by the appearance of the CO2 asymmetric stretch band in the FTIR spectrum. Thus, relative to pzc, Phe is weakly chemisorbed at negative potentials, changes orientation at potentials close to the pzc and is oxidized at positive potentials.

Electrochemical and FTIR studies of 4-cyanopyridine adsorption at the gold(111) ∣ solution interface

Abstract Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been employed to study the adsorption of 4-cyanopyridine (4-CNPy) at the Au(111) electrode surface. The vibrational spectra have been used to study the character of surface coordination and the stability of adsorbed 4-CNPy molecules. Our studies show that 4-CNPy molecules are totally desorbed from the Au(111) surface at potentials lower than −0.7 V versus SCE and that they adsorb at the gold surface at more positive potentials. At potentials lower than 0.05 V versus SCE, the adsorption has a non-dissociative character. The 4-CNPy molecules are initially oriented flat (π-bonded) on the electrode surface and reorient from the flat to a vertical state when the electrode potential approaches 0 V (SCE). When the potential is greater than 0.05 V versus SCE, the character of 4-CNPy adsorption becomes dissociative and the adsorbed molecules partially hydrolyze to form isonicotinamide (INA). The IR spectra acquired at very positive potentials indicate that 4-CNPy is oxidized at E>0.6 V versus SCE.

Quantitative studies of benzonitrile adsorption at the low-index gold single crystal electrodes

Abstract The adsorption of benzonitrile (BN) on the Au(111), Au(100), and Au(110) electrodes in aqueous solutions was investigated. The chronocoulometric technique was used to measure the charge density at the metal surface as a function of the electrode potential for BN concentrations ranging from 0 to 0.030 M. The adsorption parameters such as the film pressure, the surface concentration of the organic adsorbate, the standard Gibbs energy of adsorption (ΔGads0), and the charge flowing to the interface per one adsorbed molecule, were determined from the thermodynamic analysis of the charge density data. The analysis was carried out at constant electrode potential and at constant charge density. In addition, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) was employed to determine the orientation of BN molecules at the Au(111) surfaces. The thermodynamic and spectroscopic data indicate that BN molecules assume a flat (π-bonded) state at the negatively charged surface and progressively reorient from the flat to the tilted (N-bonded) state as the charge at the metal surface becomes more positive. At very positive potentials the character of BN adsorption becomes reactive and the adsorbed molecules partially hydrolyze to form benzamide (BA).

Efficient bacterial disinfection based on an integrated nanoporous titanium dioxide and ruthenium oxide bifunctional approach

The increasing lack of drinking water around the globe is of great concern. Although UV irradiation, photocatalysis, and electrocatalysis for bacterial disinfection have been widely explored, the synergistic kinetics involved in these strategies have not been reported to date. Herein, we report on an efficient and cost-effective strategy for the remediation of a model bacterium (E. coli), through the integration of photochemistry and electrochemistry based on a bifunctional electrode, which utilizes titanium (Ti) as the substrate, nanoporous titanium dioxide (TiO2) as a photocatalyst, and ruthenium oxide (RuO2) nanoparticles as an electrocatalyst. The nanoporous TiO2 was grown directly onto a Ti substrate via a three-step anodization process, and its photocatalytic activity was significantly enhanced by a facile electrochemical treatment. A high disinfection rate at 0.62 min-1, with >99.999% bacterial removal within 20 min was achieved using the novel TiO2/Ti/RuO2 bifunctional electrode. Complete bacterial disinfection was attained within 30 min as assessed by a spread plate method. Bacterial survival strategies, including a viable but non-culturable state of the bacteria, were also investigated during the bifunctional treatment process. The novel strategy demonstrated in this study has strong potential to be utilized for water purification and wastewater treatment as an advanced environmentally compatible technology.