Arsenic deposition onto a gold substrate was studied as a function of pH, deposition potential, and time. Deposition of less than three monolayers was observed. The rate of deposition decreased as the thickness of the deposit increased. A set of partial reactions leading to arsenic deposition was proposed. The kinetics of deposition at the most cathodic potentials corresponds to a high-field deposition mechanism.Arsenic is a semimetal which has the ability to reduce the grain size in gold electrodeposits (1-3). However, the mechanism by which this occurs is not well understood. An examination of the deposition of arsenic itself upon a gold substrate may serve to help explain the process by which this occurs.Electrodeposited arsenic is usually amorphous (4-6) but becomes crystalline when heated to temperatures greater than 250~ (4, 5). Tammann and Warrentrup (5) claimed that electrodeposited arsenic has an electrical resistance 10 ~3 times greater than crystalline arsenic. Wranglen (7) has also found that electrodeposited arsenic films have a large electrical resistance. Piontelli and Poli (8), however, attributed the behavior of arsenic during electrodeposition to a large overvoltage at the surface. This large resistance, whether due to bulk resistance or overvoltage, prevents arsenic from plating indefinitely upon substrates. It reaches a limiting thickness in most cases. When plated potentiostatically, as has been done in this paper, the deposition current gradually drops. When plated galvanostatically, the current efficiency drops continuously. Kochegarov and Lomakina (4) have noted that after being annealed, arsenic films can be plated upon to form thicker arsenic layers. This indicates that the self-limiting nature of arsenic deposition is connected to the amorphous structure of the electrodeposit.The deposition of arsenic upon gold was studied by using cyclic voltammetry and potentiostatic plating. Cyclic voltammetry was done over a range of pH values in order to determine the potential ranges over which arsenic deposition occurred. The cyclic voltammetry also revealed that there were three cathodic reactions that were taking place before arsenic deposited on the gold substrate. The potentiostatic plating was performed as a function of potential and time in order to determine the amount of arsenic deposited and the type of kinetics that the process exhibited.

ChemInform Abstract The electrodeposition of As onto a Au substrate is investigated by using cyclic voltammetry and potentiostatic plating as a function of pH, deposition potential, and time in order to find an explanation for the process by which As reduces the grain size in Au electrodeposits. The number of monolayers in the experiments shown does not exceed three. The rate of deposition decreases as the thickness of the deposit increases, because As inhibits its own deposition by forming surface layers which have a high electric resistance or a high overvoltage. A set of partial reactions leading to As deposition is proposed. At the most cathodic potentials the growth of As is governed by a high-field mechanism. Addition of As to Au plating baths reduces the grain size of the Au crystallites by masking off portions of the electrode surface and hence halting the crystallite growth at an early stage. Au crystallite growth stops and further Au deposition take place only by the nucleation of new crystallites.