Depth profiling catalysis

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

In catalysis it is important not only to be able to study the surface region of the catalyst itself, but also any adlayer that may be present. The latter may arise from adsorption or preferential segregation of one component from the bulk to the surface. The spatial distribution of the elements at the surface can be obtained from scanning AES. Experiments, in which the surface is progressively eroded, e.g., by ion-bombardment, with surface analysis by AES, XPS or ISS carried out after various times, may provide concentration-depth profiles of the chemical species. 

The elemental Pt(0) Is dispersed throughout the surface polymer as determined by depth profile analysis,(7) and a representation of the interface is given in Scheme V. According to this view there is a certain amount of Pt(0) in contact with the thin SiOx overlayer on the bulk p-type Si. This is a relevant structural feature, since direct deposition of Pt(0) onto photocathode surfaces is known to improve the efficiency for the reduction of H2O to H2> Thus, we expect that, for an interface like that depicted in Scheme V, there will be a certain amount of the H2 evolution occurring by direct catalysis of the reaction of the photoexcited electrons with H2O at the Si0x/Pt(0) interfaces. In the extreme of a uniform, pinhole-free coverage of Pt(0) on p-type Si/SiOx one expects that the photocathode would operate as a buried photosensitive interface and in fact would be equivalent to an external solid state photovoltaic device driving a photoelectrolysis cell with a Pt(0) cathode. 

The paper by Watanabe et al. is important for alloy catalysis they examined the possible enrichment of Cu in the surface layers of Cu-Ni alloys. They used Auger spectroscopy to study the escape of low- and high-energy electrons (around 100 and 700-1000 eV) and were able to make quantitative in-depth profiles, of which an example is shown in Figure 2. This enrichment was also confirmed by AES measurements for films by Benndorf et al. ... 

Each analytical technique discus.sed in this chapter has its own advantages and disadvantages, arising from both the physical processes involved and technical requirements. The theory behind each of the techniques would in it.self fill a book, whereas the present purpose was to set out briefly the depth profiling approach as it related to each. The theories of sputtering and of depth resolution have al.so been the subject of many publications. Individual applications (corrosion, semiconductors, catalysis, organic materials, etc.) will be described separately in this volume. 

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