Surface and Interface Phenomena Studied with Pd Metal-Oxide-semiconductor Structures : Adsorption, catalytic reactions, hydrogen sensing and Pd restructuring

Abstract: The electronic properties of palladium metal-oxide-semiconductor (Pd-MOS) structures are highly sensitive to hydrogen over a large pressure range. Such devices have been found useful in several applications, e.g. as a chemical sensing element in the so called electronic nose. This thesis takes its starting point in fundamental studies of the physics and chemistry of Pd-MOS structures and presents a model for its hydrogen sensing properties as well as addressing a number of questions of more general nature, relating to heterogeneous catalysis.The hydrogen sensitivity rests on the permeability of Pd to hydrogen and consecutive hydrogen adsorption at the Pd-SiO2 interface. Previous studies have shown that the hydrogen response follows a so called Temkin adsorption isotherm, i.e. the response is proportional to the logarithm of the hydrogen pressure. In this work it is shown that an electrostatic model describing the hydrogen adsorbate-adsorbate interaction at the interface, can explain this behavior as well as other, previously obtained, results. In addition to the dominating hydrogen adsorption interface state, a new state has been observed. This state appears to be located at the Pd side of the interface, in contrast to the dominating state which is located at the SiO2 side.The hydrogen sensitivity of the device has been utilized, in combination with mass spectrometry, to study hydrogen and CO coadsorption phenomena in ultrahigh vacuum (UHV). It is shown that, although CO by itself does not induce any response of the device, CO may significantly influence the hydrogen response. This is particularly evident in the presence of oxygen. It is shown that CO adsorbed on the Pd surface may act as a valve, hindering both hydrogen adsorption and desorption. Combined, real time, measurements of hydrogen desorption and absorption show that both processes may be induced simultaneously by CO exposure of an hydrogen covered surface. This phenomenon has only been indirectly observed in the past.Morphology changes of thin Pd films, evaporated on SiO2 at room temperature, have been followed, both in UHV and at atmospheric pressures. The techniques used include scanning force microscopy, transmission electron microscopy, ultraviolet photoemission spectroscopy, capacitance measurements and mass spectrometry. It has been found that Pd films with a thickness in the range 2-10 nm change their structure dramatically, even at temperatures as low as 473 K, if the surface is properly cleaned: An initially continuous film breaks up and large Pd islands are gradually formed. Carbonaceous species adsorbed on the Pd surface significantly lower the rate of restructuring and, in effect, lock the film in a certain stage of the restructuring process. The carbonaceous adsorbates can be removed by combustion with oxygen whereby the restructuring process restarts.Also, the catalytic oxidation of CO and H2 on both continuous and discontinuous Pd films has been studied in UHV. In the case of CO oxidation on a continuous Pd film, a modelling of the surface reaction kinetics was performed and good agreement with experimental results has been achieved. The discontinuous Pd film can be considered as a model catalyst. The interpretation of the experimental results from this study include phenomena occurring on the SiO2 support surface (spillover effects).