Oxidation state and valence of elements
Nature of the chemical bond and determination of bonding states by XPS
Macroscopic material properties (e.g., color, conductivity, adhesion, corrosion resistance, wettability, etc.) are significantly affected by the oxidation and bonding states of the elements present on a surface of a solid. XPS offers the possibility to investigate and quantify these chemical bonding states in detail. In this way, conclusions can be drawn on the valence of the individual elements.
Quantitative Composition of MoS2
XPS - Ideal technique for the determination of valences and bond states
A molybdenum sulfide powder (MoS2) used as a lubricant was analyzed by XPS for quantitative determination of oxidized Mo contents and average oxidation state. The identification and quantification of the elements present on the surface of the powder were first carried out on the basis of an XPS overview spectrum (results see Table 1).
In addition, detail spectra for the detected elements were recorded with high energy resolution to be able to evaluate the actual binding states quantitatively. The determination of the degree of oxidation was carried out by evaluation of the S2p and Mo3d spectra (see figure 1a and b). The S2p spectrum initially shows a characteristic S2p doublet, which, because of its energy, shows that all sulfur is present in its sulfidic form as MoS2. Based on the Mo3d spectrum, molybdenum can be detected in two bond states (MoS2 and MoO3).
An S2s signal can also be seen in figure 1b, in addition to the different Mo bonding states, which confirms the MoS2 binding of sulfur. The relative peak areas of the Mo3d signals of the MoS2 and MoO3 species, can be used to quantify the respective bond states. When the superficial C and proportional O contamination of the MoS powder already listed in table 1 are taken into account, the individual components can be quantified as listed in table 2:
The evaluation of the Mo3d spectrum make it possible to determine the MoO3 content in MoS2. According to this, about 7.9% of the molybdenum is present as Mo(VI)O3 in the powder.