Liquid Phase Photoelectron Spectroscopy Analysis of Chemical Bonding States in an Ionic Liquid
Introduction
Recently, ionic liquids have been recognized as the possible lubricants for metal/metal contacts. The wear and friction characteristics of the liquids are investigated using tribological experiments.
However, an in-depth understanding of the surface chemistry is required to interpret the data obtained from such experiments. Liquid Phase Photoelectron Spectroscopy (LiPPS), a variation of X-ray photoelectron spectroscopy (XPS), serves as a suitable analytical method for this purpose as it combines both surface sensitivity and chemical selectivity.
Detection and differentiation of various chemical bonding states in small, localized areas of the sample are critical for complete characterization of the surfaces formed during tribological experiments.
In addition, there are chances of deposition of the ionic liquid on a magnetic substrate, including steel, which, in turn, increases the analysis of demand. This needs an XPS tool that features small spot capability, high sensitivity and excellent energy resolution, but which can be achieved on magnetic samples.
The wear surface produced by the tribological action of a brass rod on a steel disc can be studied using the Thermo Scientific K-Alpha (Figure 1). The lubricant used was the ionic liquid 1-hexyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate with chemical structure shown in Figure 2.
Figure 1. Thermo Scientific K-Alpha
Figure 2. Chemical structure of 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate
Experimental
Figure 3 shows an optical image of the worn steel surface. The live Reflex optics system, unique to K-Alpha was used to acquire the image. This system allows the user to confidently choose the analysis points for large or small area XPS analysis.
A graphical marker enables the XPS probe to be precisely replicated on the live optical view. As a result, the user can easily select the most suitable probe size for the feature of interest.
Figure 3. K-Alpha live optical image of tribologically induced wear track.The ellipses indicate the positions and sizes of the analyzed areas.
The K-Alpha optical image can display the wear track on the steel surface with dark and light stripes of varying widths. The graphical probe marker helps in selecting 30 and 80µm spots to analyze the differently sized dark and light stripes.
Results
Elemental Composition of Worn Surface Lubricated with Ionic Liquid
LiPPS survey spectra (Figure 3) were acquired from the areas marked in the optical image. Survey spectra allow the elemental composition of the worn surface to be quantified at the two different analysis points (see Table 1).
Figure 4 shows the LiPPS survey spectra obtained from the marked regions in the optical image. The elemental composition of the worn surface can be quantified at two different analysis points by survey spectra (Table 1).
Figure 4. XPS spectra acquired from darker (blue) and lighter (red) stripes in the wear track of the steel disc. Spectra have been normalized and offset for clarity
Table 1. Elemental quantification of worn steel surface at two analysis points
| Atomic Concentration | ||
|---|---|---|
| Element | Light Stripe | Dark Stripe |
| P | 3.20 | 3.05 |
| C | 43.05 | 45.05 |
| N | 4.47 | 4.48 |
| O | – | 3.13 |
| F | 49.27 | 44.29 |
There were no spectral features related to the steel substrate, which suggests that the ionic liquid at each point was thicker enough to completely attenuate photoelectrons from the substrate. Therefore, it can be concluded that a minimum thickness of at least 10nm may be required for the ionic liquid layer.
The presence of oxygen in the light stripe is the major elemental difference between the two points. The other elements identified in each stripe are of identical quantities. An observable difference in the C1s peak shape suggests the chances of varying carbon chemistry in the darker and lighter stripes.
Chemical Bonding States in Worn Surface Lubricated with Ionic Liquid
The high-energy resolution of K-Alpha was used to study the carbon chemistry at the lighter and darker stripes. As shown in Figure 4, the different bonding states on the worn surface can be differentiated through the carbon spectra obtained at the two analysis points (Figure 5).
Figure 5. High energy resolution C1s spectra acquired from a) worn surface using 400 µm X-ray spot b) darker stripe and c) lighter stripe on worn surface
The peaks labeled C-C and C-C-N or N-C-N are a result of the alkyl chain and imidazolium ring in the cationic component of the ionic liquid, respectively. A small amount of the C-C peak is also due to surface contamination.
The two different chemical states of carbon in the imidazolium ring, which includes carbon bonded to single nitrogen atom and carbon bonded to two nitrogen atoms, can be distinguished by the energy resolution of the K-Alpha.
The CF3 and CF2 peaks are a result of two different carbon bonding states in the anionic fluorophosphate component of the ionic liquid. The high level of aliphatic carbon in the dark stripes is the major difference between the carbon chemistry of the pale and dark stripes.
Conclusion
The carbon chemistry and elemental composition of a worn steel surface formed in a tribological experiment can be investigated using LiPPS.
The ionic liquid 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate was coated on the surface. Different carbon chemistry was observed in different points on the worn surface, and the ionic liquid layer thickness was estimated to be a minimum of 10nm.
