A new detection method for microcalorimetry of single crystal surfaces

摘要

Determining the strength of surface chemical bonds and its variation with surface structure, coverage of adsorbates, co-adsorbates, etc. represents the most fundamental and important of all measurements in surface science, and of course this thermodynamics underlies the application of surface science to many technologies. It is also the key measurement to benchmark approximations in increasingly accurate density functional theory calculations. The traditional measurement techniques are indirect and rely on desorption, e.g., thermal desorption spectroscopy or measuring isosteric heats. Although these techniques have proven powerful for many traditional reversible adsorption systems, they are only useful if the adsorbates do not form another high-temperature phase at the desorption temperature, e.g., dissociate, cluster, diffuse into the bulk or react with co-adsorbates. Direct calorimetric measurements of adsorption heat avoid this limitation. However, because of sensitivity, calorimetry is limited to large surface area samples or thin films. While there is a long history of such measurements on heterogeneous samples, application to thin film single crystals is only a decade old. Single crystal adsorption calorimetry was pioneered by D.A. King's group at Cambridge who used IR emission from the (C coated) back side of a thin single crystal to measure the temperature rise and hence differential heat of adsorption. They have used this technique to measure a variety of reversible [e.g., CO/Ni(100)] and irreversible adsorption systems [e.g., O_2/Ni(110)]. C.T. Campbell's group at Washington have subsequently developed a single crystal calorimeter similar to that of King et al. but using a thin pyroelectric polymer ribbon which is mechanically driven to make a gentle mechanical/thermal contact to the back side of the thin single crystal film to measure the temperature rise. This detection technique has several advantages relative to that of the original King calorimeter and has led to more general applicability of microcalorimetry, e.g., to the measurement of differential heats of adsorption of metals on oxides and metals on polymers. In this issue, the article by Punckt, Bodega and Rotermund introduces a new method for detection in single crystal microcalorimetry. They show that mechanical deformation of a supported thin single crystal is caused by thermal expansion as a result of the heat released in a surface chemical reaction, and this topography change can be measured very sensitively by using the thin film as one of the mirrors in a Michelson interferometer. The authors use a lateral imaging detector to measure the mechanical deformation of the thin crystal. They demonstrate the viability of this technique by titrating a CO covered 300 nm thick Pt(110) foil with O_2 at ～10~(-5) bar. The heat released during the reaction causes a thermal expansion and mechanical buckling which is detected interferometrically with a resolution of 20 nm and giving a theoretical sensitivity of a few nJ thermal input. In fact, under certain conditions (surface temperature and pressure of O_2), propagation of mechanical deformation fronts are observed that is related to the propagation of reaction fronts. This demonstrates that this detection technique can also be used as a spatially resolving microcalorimeter. The authors compare several aspects of their detection method to those of King and Campbell.