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

Importance of the Topic

Combined X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy play complementary roles in surface analysis, offering elemental/chemical state determination and molecular structural identification. Integrating these techniques within a single platform eliminates sample transfer, reduces contamination risk, and ensures co-localized data acquisition, enhancing reliability and efficiency in material characterization.

Study Objectives and Overview

This work evaluates the Thermo Scientific Theta Probe Angle-Resolved XPS (ARXPS) System integrated with the DXR3 Flex Raman Spectrometer for coincident analysis of mineral oxide species. The study focuses on calcium carbonate polymorphs (calcite, aragonite) and titanium dioxide polymorphs (anatase, rutile), demonstrating simultaneous surface cleaning, elemental analysis, vibrational spectroscopy, and quantitative phase determination.

Methodology and Instrumentation

Samples were mounted in a unified UHV chamber where the XPS and Raman probes target the same location. XPS survey and high-resolution spectra identified elemental composition and chemical states, while argon cluster sputtering (Ar1000+ at 6 kV for CaCO3; Ar2000+ at 4 kV for TiO2) removed surface contaminants without substrate damage. Coincident Raman measurements provided vibrational fingerprints of crystal lattice modes and polymorph-specific peaks. Instrumentation Used:

• Theta Probe ARXPS System (Thermo Scientific)
• DXR3 Flex Raman Spectrometer (Thermo Scientific)
• MAGCIS Monatomic and Gas Cluster Ion Source

Main Results and Discussion

In calcium carbonate analysis, XPS confirmed stoichiometric CaCO3 after cluster cleaning, removing silicon, sodium, and organic carbon. However, XPS alone could not distinguish calcite from aragonite. Raman spectra revealed distinct low-frequency lattice modes correlating with crystal symmetry, enabling polymorph differentiation. In TiO2 studies, subtle differences in XPS valence band shapes between anatase and rutile limited direct quantification, whereas Raman peak positions, notably the 142 cm-1 anatase mode, allowed clear identification. Nonlinear least squares fitting of mixed-sample Raman spectra yielded accurate anatase:rutile ratios, facilitated by TQ Analyst Software.

Practical Benefits and Applications

This multimodal platform streamlines workflows by eliminating instrumental transfers, ensuring data from identical sample regions, and reducing handling-induced artifacts. It offers robust surface cleaning and stoichiometry verification via XPS, coupled with rapid polymorph identification and phase quantification via Raman, applicable to catalysis, photovoltaics, biomineralization studies, geological analysis, and quality control.

Future Trends and Opportunities

Further developments may integrate additional spectroscopic or microscopic techniques for richer multimodal datasets, implement advanced automation for co-incident data acquisition, and expand in situ or operando capabilities. Enhanced software toolkits could streamline quantitative phase analysis, supporting materials development in energy, environmental, and biomedical fields.

Conclusion

The coincident XPS-Raman platform delivers synergistic insights into surface chemistry and molecular structure that neither technique alone can achieve fully. By aligning analyses at the same region and employing gentle cluster sputtering, it provides a powerful, efficient, and reliable tool for comprehensive material characterization.

Reference

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3. White WB. The carbonate minerals. In: Farmer VC, editor. The Infrared Spectra of Minerals. London: Mineralogical Society; 1974. p. 227–284.