Capabilities of static TOF-SIMS in the differentiation of first-row transition metal oxides

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyses were performed on the first-row transition metal oxides from scandium to zinc in positive and negative detection modes. The nature of the numerous M(x)O(y)(+/-) ionic species generated by 15 keV Ga(+) primary ion bombardment allows the identification of a given metal-oxygen system. To identify the metal valence in the oxide under investigation, several procedures were investigated: the detection of specific and characteristic ions, the use of ion abundance ratios and the use of a valence model. Owing to their importance in many fields of materials science, each of these speciation methodologies was evaluated for the differentiation of vanadium, titanium, chromium, manganese, iron, cobalt and copper oxides. Trivalent-hexavalent chromium distinction was first intensely investigated because it really corresponds to a model system for inorganic speciation. For each series of metal oxides, the more pertinent speciation criteria were then systematically tested. The limitations of the proposed methodologies are discussed. Their use is made complicated when pollutants or a superficial oxide layer, with a stoichiometry different from that of the bulk, are present. Finally, thermodynamic considerations relative to the stability of the M(x)O(y)(+/-) ions may also modify the relationship between the analyzed oxide and the observed positive and negative secondary ion mass spectra.