In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were employed to examine the underpotential deposition (UPD) of cadmium on a rhodium(111) electrode in sulfuric and hydrochloric acids. The (bi)sulfate and chloride anions in the electrolytes played a main role in controlling the number and arrangement of Cd adatoms. Deposition of Cd along with hydrogen adsorption occurred near 0.1 V (vs reversible hydrogen electrode) in either 0.05 M H2SO4 or 0.1 M HCl containing 1 mM Cd(ClO4)2. These coupled processes resulted in an erroneous coverage of Cd adatoms. The process of Cd deposition shifted positively to 0.3 V and thus separated from that of hydrogen in 0.05 M H2SO4 containing 0.5 M Cd2+. The amount of charge (80 microC/cm2) for Cd deposition in 0.5 M Cd2+ implied a coverage of 0.17 for the Cd adatoms, which agreed with in situ STM results. Regardless of [Cd2+], in situ STM imaging revealed a highly ordered Rh(111)-(6 x 6)-6Cd + HSO4- or SO42- structure in sulfuric acid,. In hydrochloric acid, in situ STM discerned a (2 x 2)-Cd + Cl structure at potentials where Cd deposition commenced. STM atomic resolution showed roughly one-quarter of a monolayer of Cd adatoms were deposited, ca. 50% more than in sulfuric acid. Dynamic in situ STM imaging showed potential dependent, reversible transformations between the (6 x 6) Cd adlattices and (square root 3 x square root 7)-(bi)sulfate structure, and between (2 x 2) and (square root 7 x square root 7)R19.1 degrees -Cl structures. The fact that different Cd structures observed in H2SO4 and HCl entailed the involvement of anions in Cd deposition, i.e. (bi)sulfate and chloride anions were codeposited with Cd adatoms on Rh(111).

The current-voltage curve of an atomically flattened hydrogen-terminated p-Si(111) surface is studied in 0.1 M ammonium borate solution in a potential region from the onset of the anodic current to about 3 V vs. Ag/AgCl. Three anodic current peaks are observed in this potential region. A deconvolution of the current-voltage (I-V) curve revealed that these three peaks are related to the oxidation of three Si monolayers from the surface. The current due to the oxidation of the first monolayer consists of two components with different peak potentials. Supported by Fourier transform infrared measurements, the component at the lower potential is attributed to the oxidation of Si-Si back bonds and the one at higher potential to the oxidation of Si-H bonds. The shape of the I-V curve is sensitive to the surface pretreatment, and we consider the shape to be a good indicator of the Si surface conditions. © 2003 The Electrochemical Society. All rights reserved.

In this study, we measured the electrochemical oxidation currents on n-Si(111) surfaces at a potential near the flat-band potential. The current became small when the surface was treated with oxygen-free water, which is effective for flattening the Si(111) surface, before the electrochemical measurement. This current was attributed to the oxidation of Si atoms on step and kink sites, and was concluded to be a good measure of the structural perfection of Si(111) surfaces.

The electrochemical current-voltage curve of a Si͑111͒ surface measured in an aqueous solution is very sensitive to the surface condition. Using this property, we analyzed the Si͑111͒ surface after chemical oxidation by H 2 O 2 . Thermodynamically, Si-Si back bonds of surface Si-H bonds are oxidized more easily than the Si-H bonds. However, when the surface was atomically fattened and hydrogen-terminated, the oxidation rate of the Si-Si back bonds was lowered and the surface Si-H bonds and Si-Si back bonds were oxidized simultaneously. The lowered reactivity of the Si-Si back bonds was attributed to the hindrance of the approach of H 2 O 2 molecules to the Si-Si back bonds by the Si-H bonds regularly stretching perpendicular to the surface. Fourier transform infrared ͑FTIR͒ spectra showed that the Si-Si back bonds on step sites were oxidized very easily, and this was attributed to easy access of H 2 O 2 molecules to the Si-Si bonds on the step sites. These results indicate that the regular structure of the atomically flattened Si͑111͒ surface is the reason for its good stability against chemicals.