Electric arc generated (Bredig) palladium nanoparticles: Surface analysis by X-ray photoelectron spectroscopy for samples prepared at different pH

Abstract

Nanoparticle (sol, colloid) science attracts increased interest these days due to the special optical and magnetic properties possessed by the particles. The current research activity is supplementing the older body of research work in this area from the early 20th century with more sophisticated and detailed characterizations of sol particles. For example, vaporization-condensation methods were popular early preparative routes to many metal sols [1]. One method involved striking an unprotected DC arc between 2 metal poles in a liquid, most frequently water (the “Bredig Method”). “Bredig” sols were judged to be impure due to impurities from the concurrent electrolytic decomposition of the electrolyte and oxidized material [2] thought to form during arcing. Such results, however, were not proven directly till later times when modern (surface analytical) XPS techniques became available [3]. In this study, modern XPS techniques are used to investigate the surface speciation of Bredig palladium sols as a function of pH of the aqueous medium in which the palladium arcs are struck. The production of nanoscale Pd materials is of obvious interest for catalytic purposes in the motor vehicle industry where Pd is used widely in catalytic convertors. The electric circuit diagram for the power supply used for generating the Bredig palladium sols is illustrated in Fig. 1. A three-phase AC electric power supply rectified to 200 V DC (with a resistive ballast of 40 ohms to control the current level) was used. Positive and negative pole sockets were incorporated into the unit to allow switching of electrodes so that different metals could be arced. The unit also incorporated a keylock and interlock system for safety purposes. An electrical fan on an independent 230 V mains AC circuit cooled the unit during operation. The pole electrodes were BNC plugins with insulating retractable shrouds. The entire electrode save for the (active) pole at the end was encased in heat shrunk insulation with a silicone seal plug at the end to prevent fluid seepage into the interior of the glass tubing housing the wiring. Short lengths of BDHL.R. grade Pd wire of 0.91 mm diameter and of 99.97% purity that were soldered onto the copper wire within the glass sleeving of each electrode was used for the palladium poles. The arcing vessel was a custom designed three-necked flask which had two of its necks arranged (see Fig. 1b) to allow the two