Analysis of Molybdenum Sulfide (MoS2)

A molybdenum sulfide powder (MoS2), used as a lubricant, was analyzed by XPS for the quantitative determination of the fraction of oxidized Mo and the average oxidation state. For the identification of the elements present on the powder surface first an XPS overview spectrum was recorded. Subsequently, detail spectra with a better energy resolution were acquired for the detected elements and quantified (see table 1).

Mo

S

O

C

19.0

34.9

7.0

39.1

Table1: Element concentrations (all data in atom-%)

 XPS Detailspektren S2p und Mo3D

Figure 1: S2p-fit ​​spectrum (left) and Mo3d-fit spectrum with S2s signal (right)

The degree of oxidation could be determined by a detailed analysis of the S2p- and Mo3d spectra (figure 1). The S2p spectrum initially shows a characteristic S2p doublet. The position along the energy axis shows  that all sulfur is bound in the sulphidic form as MoS2. Based on the Mo3d spectrum molybdenum can be detected in two binding states (MoS2 and MoO3).

In addition to the Mo peaks, in this energy range an S2s signal can be recognized, which reaffirms the conclusions about the binding state of sulfur as MoS2. The relative peak areas of the Mo3d signals of the MoS2 and MoO3 species can be used to quantify these binding states. Apart from the MoO3 and MoS2, the powder has a surface contamination, containing C and part of the O. This results in the quantitative allocation of the individual components, listed in table 2 (compare to table 1). 

Assignment

Powder surface

MoS2

17.5 at-% Mo

34.9 at-% S

MoO3

1.5 at-% Mo

4.5 at-% O
Contamination

39.1 at-% C

2.5 at-% O

Table 2: Quantitative assignment
The evaluation of the Mo3d spectrum enables the determination of the MoO3 content in MoS2. According to this analysis approximately 7.9% of the molybdenum is present as Mo(VI)O3 on the powder.