Surface Modified Electrodes and Their Reactivity

Various aspects of the adsorption of Cd²⁺ on self assembled monolayers (SAM) of 4-heptadecylpyridine (HDpy) and 7-tridecyl-4-methyl-1,10-bipyridine (TMbipy) supported on an octadecylsilane (ODS)-modified Ge prism have been examined both ex situ and in situ using attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). Analysis of spectra acquired ex situ for HDpy/ODS/Ge and TMbipy/ODS/Ge before and after exposure to Cd²⁺, yielded values for the fraction of uncoordinated pyridine, θ(py_un), and uncoordinated bipy, θ(bipy_un) of ca. 0.5 and ca. 0.1, respectively. The adsorption isotherm was obtained for HDpy/ODS/Ge in contact with Cd²⁺-containing solutions in the range 10 nM < [Cd²⁺] < 0.1 mM.

The affinity of Cd²⁺ toward commercial carboxyl-terminated iron oxide beads (C-TIOB) and synthesized undecanoic acid (UA)-SAM functionalized hematite nanoparticles were investigated using anodic stripping voltammetry (ASV) as a function of both pH and [Cd²⁺] in the µM range. Adsorption data collected as a function of [Cd²⁺] and pH revealed a marked increase in the binding constant of Cd²⁺ for carboxylate ions upon attachment to the solid support, over values predicted from standard solution phase equilibria. The number density of carboxylic/carboxylate groups on the surface of synthesized UA-SAM functionalized hematite nanoparticles, which was determined by titration, was consistent with a closed-packed monolayer.

Tetrahedral amorphous carbon containing nitrogen (taC:N) films have been deposited on semiconductor (SC) electrodes (n-GaAs, n-CdS, n-CdSe) using an ion-filtered cathodic vacuum arc (FCVA) carbon source under high vacuum. The electrochemical properties of the specimens denoted as taC:N|SC were studied in dark and under illumination. Whereas the particular bare SC electrodes were found to undergo corrosion under illumination in neutral solutions, the corresponding taC:N|SC counterparts did show protection, albeit limited, against surface degradation. Depending on the specific SC, the taC:N|SC junctions were found to be rectifying or ohmic. The photocurrent and the photovoltage of taC:N|n-GaAs were proportional to the intensity and to the logarithm of the intensity of illumination, respectively. The deposition conditions were found to influence markedly the properties of the resulting taC:N|SC films. Unfortunately, none of the taC:N|n-GaAs specimens examined showed protection against photocorrosion in alkaline solution.