[en] We have studied the correlation between the chemical state and the oxygen-sensing properties of an iron oxide thin film using a setup that allows simultaneous sensor resistance measurements and X-ray photoelectron spectroscopy (XPS) data acquisition. The gas exposures were performed at the highest operating pressure of the XPS spectrometer at a controlled sample temperature which allows direct comparison between the sensor response and the chemical state of the surface. The iron oxide film was modified by a sequence of argon ion sputtering steps and the induced changes in the chemical state, resistance, and sensitivity to oxygen were investigated. The sputtering was found to reduce the iron from the Fe³⁺ to the Fe²⁺ state and to decrease the sensor resistance. The measured sensitivity to oxygen first increased by a factor of two but then collapsed to its original level. The mechanism for oxygen sensing was found to be filling of the oxygen vacancies in the lattice. The effect of the sputtering on the resistance and sensitivity could be explained first with an increase in the density of oxygen vacancies and then, as the iron became more reduced, with an increase in the p-type conductivity

[en] A statistical model for sintering of solution-processed electrically conducting nanoparticle structures is developed. The model considers thermal expansion of the particles under Joule heating as the driving force of the process. The results are used to explain the fast resistance transition observed for the recently reported rapid electrical sintering process. A comparison with experimental results shows good agreement for the kinetics of the process. A heat-equation solution is also derived for a generic geometry of a printed conductor. This provides a basis for further refinements of the model to take other driving mechanisms, such as diffusion and inter-particle forces, into account. The results of this paper help in developing quantitative understanding of the physical processes that are relevant in nanoparticle sintering.

[en] We have investigated chromium layers evaporated onto a thin alumina film at room temperature. The oxidation and reduction behavior of this model catalyst was compared to atomic layer deposition (ALD) and impregnated alumina supported catalysts using X-ray photoelectron spectroscopy (XPS) with a detailed analysis method utilizing asymmetric peak shapes to represent both metallic and oxidic states. The ALD and impregnated catalysts were measured after calcination in air and after reduction with several gases at 850 K. Both catalysts show Cr³⁺ and Cr⁶⁺ species after calcination and mostly Cr³⁺ after reduction. The chromium layers deposited in vacuum show initially small partial oxidation due to the interaction with the oxygen terminated alumina film. These model catalysts can be oxidized in vacuum to Cr³⁺ species but not to higher oxidation states. The model catalysts were also subjected to calcination and reduction treatments after deposition in vacuum. Under these conditions the model systems exhibit similar oxidation/reduction behavior as the supported catalysts. Photoreduction of Cr⁶⁺ during the measurements was also studied and found to be very slow having a negligible effect on the results