Advantages of coincident XPS-Raman in the analysis of mineral oxides species

Significance of the Topic

X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy are widely used techniques in surface and materials analysis. Combining them in a single instrument removes sample transfer steps, reduces contamination risk, and ensures data are collected from exactly the same spot. This multimodal approach enhances confidence in both chemical composition and molecular structure determinations.

Study Objectives and Overview

This work presents the integration of a Thermo Scientific Nexsa XPS system with an iXR Raman spectrometer. The primary goals are to demonstrate coincident XPS-Raman measurements on mineral oxides, validate simultaneous analysis of polymorphs, and highlight the efficiency gains achieved by in-vacuum alignment of the two techniques.

Methodology and Instrumentation

Sample analysis was performed in ultra-high vacuum using:

• Thermo Scientific Nexsa XPS system for elemental and chemical state analysis.
• Thermo Scientific iXR Raman spectrometer for molecular vibrational information.
• MAGCIS gas cluster ion source for gentle surface cleaning with Ar1000+ and Ar2000+ clusters to remove adventitious carbon without altering underlying chemistry.

Data acquisition included survey and high-resolution photoemission spectra (C 1s, O 1s, Ca 2p, Ti 2p, valence band) interleaved with cluster cleaning, followed by Raman spectra recorded from the same location.

Main Results and Discussion

Calcium carbonate analysis:

• XPS surveys of natural calcite and aragonite revealed surface contamination by silicon, sodium and aliphatic carbon. Cluster cleaning restored stoichiometric CaCO3 and removed contaminants.
• XPS spectra could not differentiate calcite and aragonite polymorphs, but verified surface integrity for subsequent Raman.
• Raman spectra showed distinct lattice-mode peaks: calcite displayed fewer, more symmetric bands, while aragonite exhibited multiple low-frequency modes due to its orthorhombic structure.

Titanium dioxide polymorph quantification:

• XPS valence band shapes of anatase and rutile differed subtly, making quantification challenging by XPS alone.
• Raman spectra exhibited clearly separated reference peaks (e.g., 142 cm⁻¹ for anatase), enabling straightforward distinction and quantification of anatase:rutile ratios in mixed samples via non-linear least squares fitting.
• Thermo Scientific TQ Analyst software facilitated development of a quantitative Raman method based on reference spectra.

Benefits and Practical Applications

The coincident XPS-Raman platform offers:

• Guaranteed spatial correlation of chemical and molecular data.
• Reduced sample handling and analysis time.
• Improved reliability when studying non-uniform or complex surfaces.

This methodology is valuable in fields such as mineralogy, catalysis, photovoltaics, biomineralization studies, and quality control in materials processing.

Future Trends and Potential Applications

Advances may include:

• Integration with additional modalities (e.g., infrared, UV-vis spectroscopy) for richer datasets.
• In situ and operando studies of reaction processes.
• Automated data fusion and machine-learning algorithms for rapid material classification.
• Expansion to soft materials, thin films, and biological interfaces.

Conclusion

Coincident XPS-Raman analysis unites quantitative surface chemistry with definitive molecular identification in a single step. This synergy enhances data confidence, streamlines workflows, and opens new possibilities for comprehensive materials characterization.

Reference

• Kontoyannis C.G., Vagenas N.V. Calcium carbonate phase analysis using XRD and FT-Raman spectroscopy. Analyst. 2000;125:251–255.
• Addadi L., Joester D., Nudelman F., Weiner S. Mollusk shell formation: concepts for understanding biomineralization processes. Chem. Eur. J. 2006;12:980–987.
• White W.B. The carbonate minerals. In: Farmer V.C., editor. The Infrared Spectra of Minerals. London: Mineralogical Society; 1974. p. 227–284.