Advantages of Coincident XPS-Raman in the Analysis of Mineral Oxides Species

XPS is a very versatile method that has found extensive use in numerous application areas, from aerospace materials to contact lenses. XPS is unique in that it can quantify the chemical and elemental composition of a material’s surface with superior selectivity; the standard information depth of XPS being less than 10 nm.

Raman spectroscopy is utilized in a number of similar application areas as it requires minimal sample preparation, is nondestructive, offers information on molecular structure, and allows users to identify materials swiftly, owing to wide-ranging spectral libraries. The implementation of XPS and Raman with other analysis methods is well defined; XPS has an extensive history of complementary use with related UHV analysis methods, such as Auger electron spectroscopy, UV photoelectron spectroscopy and ion scattering spectroscopy. In these cases, extra equipment is added to the spectrometer to provide co-incident, complementary information.

Figure 1. Thermo Scientific iXR Raman Spectrometer coupled to a Thermo Scientific Nexsa XPS System.

Raman spectroscopy is also often used along with other analytical methods to provide information on chemical environment and molecular structure. However, coincident XPS and Raman spectroscopy has been impossible. Instead samples have had to be shifted between instruments; which not only increases the amount of time needed to obtain data, but also adds a degree of indecision as to whether the analysis has been gathered from the same region of the sample.

To overcome these issues, the Thermo Scientific™ Nexsa™ spectrometer has been combined with the Thermo Scientific™ iXR™ Raman Spectrometer (Figure 1) to provide a multi-modal analysis system. The system aligns the XPS analysis position precisely with the Raman analysis position, guaranteeing that the data is gathered from the same position.

The coincident XPS-Raman eliminates any requirement to shift the sample from one instrument to the next between analyzes, which reduce extra sample handling and exposure to varied conditions which can cause sample degradation or contamination.

Analysis of titanium dioxide (TiO₂) and calcium carbonate (CaCO₃) polymorphs serve as outstanding examples of the robust information that can be collected from performing XPS and Raman spectroscopy simultaneously. Both compounds occur in multiple crystalline forms, which can be differentiated using Raman but not easily by XPS. However, establishing the quantity and type of contamination existing in naturally occurring mineral samples is hard using Raman in isolation, as is recognizing compounds that are not found within a spectral library.

CaCO₃ occurs naturally in three varying polymorphs: Calcite, Vaterite and Aragonite, with Calcite and Aragonite being the two most abundant. Calcite has a trigonal crystal structure and is the most thermodynamically stable form of calcium carbonate. Aragonite possesses an orthorhombic crystal structure, is less thermodynamically stable, gradually changing to Calcite under ambient conditions, and forms, geologically, at high temperature and pressure.

The differences in crystal structure cause difference in the physical properties of the different polymorphs, such as solubility. Establishing the causes behind these variances is not only vital to understanding geological formations, but it is also crucial in a diverse range of applications areas, such as pancreatic calcification¹, industrial scale formation and biomineralization². Mixtures of Calcite and Aragonite form naturally in marine molluscs and the water pressure, temperature and salinity all intensely influence the proportion of each polymorph found in the shells of diverse species.

TiO₂ is one of the most analyzed materials in surface science, because of its availability, low toxicity and extensive range of applications, such as catalysis, photovoltaics and self-cleaning windows. As a result of this occurrence throughout the field, it is frequently referred to as the prototypical metal oxide surface. Whilst a number of polymorphs of TiO₂ are known to exist, only two occur naturally in abundance: Anatase and Rutile. Rutile-TiO₂ is the more thermodynamically stable form but Anatase-TiO₂ is more energetically promising when forming nanoparticles at atmospheric pressure and temperature, conditions which are utilized when growing films by solution based processes in instruments such as solar cells.

Furthermore Anatase-TiO₂ has formerly been recognized as more photocatalytically active than Rutile-TiO₂, however latest research shows that the highest photovoltaic efficiencies are realized in devices that contain a blend of both polymorphs.

Experiment and Results

XPS spectra across an extensive range of binding energies, known as survey spectra, were obtained from natural crystals of Aragonite and Calcite (see Figure 2) to establish the elemental composition, as anticipated for naturally occurring crystals, surface contamination was noticed in the form of sodium and silicon. High resolution spectra of each photoemission peak were then attained to allow determination of chemical states present at the surface; the C 1s photoemission peak indicates the existence of aliphatic carbon contamination besides the carbonate peak.

Repeated cycles of sputtering with Ar1000+ clusters at 6kV generated by the Thermo Scientific™ MAGCIS™ (Monatomic and Gas Cluster Ion Source) removed aliphatic carbon contamination from the surface as shown in Figure 3, and reacquisition of the survey spectra confirms the elimination of sodium and silicon contamination, leaving stoichiometric CaCO₃.

Figure 2. Photographic image of aragonite (right) and calcite (left) crystals.

Figure 3. Removal of surface contamination on CaCO₃ crystals with argon ion gas clusters.

As can be seen from the overlay of the survey and valence band spectra attained from the cluster-cleaned aragonite and calcite crystals (Figure 4), it is not possible to differentiate between the two different polymorphs of CaCO₃ using XPS. However the combination of XPS and cleaning with argon ion gas clusters does establish that the surface possesses the precise stoichiometry and does not have any contamination, thus allowing Raman spectra to be obtained with absolute confidence in the chemical and elemental composition of the region of interest.

Figure 5 illustrates the Raman spectra acquired from the two different crystalline forms of CaCO₃. The higher shifted peaks match what are mentioned as internal modes related to the carbonate anion. While a slight shift can be noticed in some of these peaks they are similar in both polymorphs. The peaks in the lower shifted region are because of lattice modes and rely on the way the carbonate anions are arranged in relationship to each other in the crystal structures. The Aragonite structure’s lower symmetry results in many more peaks in this region of the Raman spectrum. The precise assignments of these varied vibrational modes have been addressed elsewhere.³

Using Raman spectroscopy, it is also possible to establish the ratio of polymorphs in mixed samples. By using polymorphically pure samples, of TiO₂, as references it is possible to formulate a technique for establishing the percentage of each polymorph present. A total of five powders were tested using coincident XPS-Raman: Pure Anatase-TiO₂, pure Rutile-TiO₂ and three mixed powders.

Survey spectra, high resolution C 1s, O 1s, Ti 2p and valence band photoemission spectra were repeatedly acquired from all the TiO₂ powders at the same time as performing cleaning cycles with Ar₂₀₀₀+ gas clusters at 4 kV to eliminate surface carbon contamination. The application of argon ion gas clusters allows surface contamination to be eliminated without destroying the chemistry of the underlying substrate, as demonstrated by the lack of a metallized titanium peak at 455 eV, this emerges when etching with monatomic argon due to the preferential sputtering of oxygen, forming sub-stoichiometric TiO₂-x.

Figure 4. XPS spectral overlay (cluster-cleaned).

Figure 5. Overlay of the Raman spectra obtained from the CaCO₃ crystals.

Slight differences can be seen in the valence band spectral shapes obtained from the pure Anatase-TiO₂ and Rutile-TiO₂ powders using XPS (see Figure 6), however the variances are subtle, making determination of the relative proportion of each polymorph hard (Figure 7). However, the distinctly different Raman shift peak positions seen in the pure Anatase-TiO₂ and Rutile-TiO₂ powder spectra allow easily differentiating between the Anatase and Rutile polymorphs (Figure 8). Additionally the spectral profiles can be used as references for non-linear least squares fitting of the Raman spectra obtained from the mixed powders (Figure 9), enabling fast and direct determination of the Anatase:Rutile ratio of the three mixed powder samples.

It is also possible to use the 142 cm^-1 peak in the Raman spectrum of Anatase as the foundation of a quantitative technique for establishing the percentage of Anatase in a blend of Anatase and Rutile. The Thermo Scientific™ TQ Analyst™ software offers a convenient way to formulate these types of quantitative approaches.

Figure 6. XPS valence band comparison – Pure TiO₂ powders.

Figure 7. XPS valence band comparison – Mixed TiO2 powders.

Figure 8. Comparison of the Raman spectra of the pure TiO₂ powders.

Figure 9. Comparison of the Raman spectra of the mixed TiO₂ powders.

Summary

In conclusion, XPS is used for quantitative determination of both chemical and elemental composition for any solid material compatible with ultra-high vacuum. While Raman spectroscopy is used for identification of referenced compounds by careful spectral matching using spectral databases and searching algorithms. The combination of Raman and XPS on a single instrument, Nexsa and iXR, allows more robust analysis of a material than either technique in isolation, with the purity, cleanliness and stoichiometry of a sample established using XPS, and identification and quantification of molecular structures to be established using Raman spectroscopy.

As both techniques are aligned to the same position within the vacuum system all the time consuming aspects of locating the same analysis point when shifting between instruments is removed, delivering unquestionable certainty that the information obtained has all come from the same region of sample, which is particularly useful when examining non-uniform samples.

References and Further Reading

1. Christos G. Kontoyannis and Nikos V. Vagenas, Calcium carbonate phase analysis using XRD and FTRaman spectroscopy, Analyst, 2000, 125, 251-255.
2. Lia Addadi, Derk Joester, Fabio Nudelman, and Steve Weiner, Mollusk Shell Formation: A Source of New Concepts for Understanding Biomineralization Processes, Chem. Eur. J. 2006, 12, 980-987
3. W.B. White, The carbonate minerals. Chap. 12, In: V.C. Farmer (ed.), The Infrared Spectra of Minerals, Mineralogical Society of London, 1974, p. 227-284.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).

For more information on this source, please visit Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).