These effects are responsible for the color change in the analysis area seen in Figure 3. Figure 4 shows that most of the iron (III) surface atoms in hematite are rapidly reduced to iron (II), and some atoms are completely reduced to metallic iron. The analysis was repeated several times, and the solid line was fitted to the average of the multiple analyses. A plot of iron oxidation states as a function of ion sputtering time if shown in Figure 4. The dark square is the analysis area, where the sample was analyzed after sequentially sputtering with a 1.0 kV argon ion beam. The red hematite powder was pressed into indium foil, and then analyzed in a Physical Electronics Quantum 2000 XPS system. These chemical changes severely limit the use of ion sputtering in the study of oxide films on metal surfaces.įigure 3 shows a sample of hematite (Fe 2O 3) powder that was analyzed to show the effects of ion sputtering on iron oxidation states. Some of the oxide may even be fully reduced to elemental metal.
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As outlined in Reference 3, higher valent oxides tend to reduce to common suboxides, although a mixture of oxidation states is typically found even after very long sputtering times. In the case of transition metal oxides, oxygen atoms typically sputter at a higher rate than the metal atoms, leading to reduction of the metal’s oxidation state. In a material with multiple elements, elements may sputter at different rates, and hence change the stoichiometry of the analyzed surface. However, using an ion beam for cleaning or profiling can change the composition of a material through effects such as preferential sputtering and sputter reduction (2,3).
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Depth profiling is used extensively for analysis of layered samples. Using an ion beam to sequentially etch material from a surface during analysis can also produce a depth profile, which shows the distribution of elements from the surface into the bulk of a sample. This process is known as ion beam sputtering. Cleaning in situ in a vacuum chamber can be performed by using an ion beam to etch material from the surface of a solid sample. However, in many cases, the outermost surface of a sample may be altered or contaminated, and needs to be cleaned to expose pristine material for analysis. The study of transition metal oxidation states on solids surfaces provides information on many important industrial processes, including corrosion, catalysis and lubrication. The measured spectrum is well fit by a linear combination of the reference spectra from Figure 1, and indicates that the analyzed area contains 28☒% Fe(0), 41±5% Fe(II), and 32☖% Fe(III). Figure 2 shows an example linear peak fit of a mixed iron oxidation state sample. The spectra exhibit a variety of features that allow the iron metal, iron (II) and iron (III) species to be distinguished. Figure 1 shows examples of iron XPS spectra from reference compounds. X-ray photoelectron spectroscopy (XPS also known as ESCA – electron spectroscopy for chemical analysis) is a surface analysis technique that can be used to distinguish the oxidation state of transition metals(1).