Using a high-stability quartz-crystal microbalance to measure and model the chemical kinetics for gases in and on metals: oxygen in gold

摘要

This paper describes the use of a high-stability quartz-crystal microbalance(QCM) to measure the mass of a gas absorbed on and in the metal electrode onthe quartz oscillator, when the gas pressure is low and the gas can beconsidered as rigidly attached to the metal, so viscosity effects arenegligible. This provides an absolute measure of the total mass of gas uptakeas a function of time, which can be used to model the kinetic processesinvolved. The technique can measure diffusion parameters of gases in metalsclose to room temperature at gas pressures much below one atmosphere, asrelevant to surface processes such as atomic layer deposition and model studiesof heterogeneous catalysis, whereas traditional diffusion measurements requiretemperatures over 400oC at gas pressures of at least a few Torr. A strongaspect of the method is the ability to combine the bulk measurement of absorbedmass by a QCM with a surface-sensitive technique such as Auger electronspectroscopy in the same vacuum chamber. The method is illustrated using atomicoxygen, formed under O2 gas at 6x10-5 Torr in the presence of a hot tungstenfilament, interacting with the gold electrode on a QCM crystal held at 52 to120oC. Some of the incident oxygen forms a surface oxide which eventuallyblocks more uptake, and the rest, about 80%, indiffuses. Surprisingly, the rateof oxygen uptake initially increases with the amount of oxygen previouslyabsorbed; therefore, the measured oxygen uptake with time is reproducible onlyif pre-adsorption of oxygen conditions the sample. Temperatures above 130oC arenecessary for measurable thermal desorption, but all the oxygen can be removedby CO scavenging at all temperatures of these experiments. Simple kineticmodels are developed for fitting the experimental data to extract relevantparameters.