X-ray photoelectron spectroscopy (XPS) is a highly versatile technique that is extensively used in various application areas, ranging from aerospace materials to contact lenses. XPS is known to be unique because it can measure the chemical and elemental composition of the surface of a material with extreme selectivity; the standard information depth of XPS is less than 10 nanometers.
Raman spectroscopy is non-destructive, requires little sample preparation, provides information on molecular structure, and allows users to rapidly identify materials due to extensive spectral libraries. As a result, this technique is employed in many similar application areas.
Raman and XPS have been implemented with other analysis techniques and this approach is well established; in fact, XPS has a long history of complementary use with related UHV analysis techniques, such as ion scattering spectroscopy, Auger electron spectroscopy, and UV photoelectron spectroscopy.
In such cases, more equipment is added to the spectrometer in order to provide co-incident, complementary data.
Figure 1. Thermo Scientific iXR Raman Spectrometer coupled to a Thermo Scientific Nexsa XPS System.
In addition, Raman spectroscopy is often used in tandem with other analytical techniques in order to provide information on chemical environment and molecular structure. However, it was not possible to achieve coincident XPS and Raman spectroscopy and instead samples had to be transferred between instruments. However, this tends to increase the amount of time needed to obtain data and also adds some amount of uncertainty as to whether the analysis has been collected from the same sample region.
In order to overcome these problems, a multi-modal analysis platform is provided by integrating the Thermo Scientific™ Nexsa™ spectrometer with the Thermo Scientific™ iXR™ Raman Spectrometer (Figure 1). The system exactly aligns the XPS analysis position with the Raman analysis position, making sure that the data is collected from the same position.
In the coincident XPS-Raman, there is no need to transfer the sample from one instrument to the next between analyses and hence, additional sample handling and exposure to different conditions is minimized which can otherwise lead to contamination or degradation of sample.
Analysis of titanium dioxide (TiO2) and calcium carbonate (CaCO3) polymorphs serves as good examples of the powerful data that can be obtained by performing XPS and Raman spectroscopy simultaneously. Both compounds – TiO2 and CaCO3 – occur in multiple crystalline forms, which can be identified using Raman but not easily by the XPS technique. However, when Raman spectroscopy is used in isolation, it is difficult to determine the type and quantity of contamination present in naturally occurring mineral samples. It is also difficult to identify the compounds that are not contained within a spectral library.
Calcium carbonate occurs naturally in three different polymorphs, such as Vaterite, Aragonite and Calcite, with Calcite and Aragonite being the most abundant ones. Calcite, the most thermodynamically stable form of calcium carbonate, has a trigonal crystal structure. Aragonite, the less thermodynamically stable form of calcium carbonate, has an orthorhombic crystal structure. It gradually changes to Calcite under ambient conditions, and forms, geologically, at high pressure and temperature.
The variations in crystal structure change the physical properties of the different polymorphs, for example solubility. It is important to establish the causes behind these differences to understand geological formations; these causes also have to be established in a wide range of applications areas, such as pancreatic calcification1, biomineralization2, and industrial scale formation. Mixtures of Aragonite and Calcite occur naturally in marine molluscs, and the pressure, temperature, and salinity of water all strongly affect the proportion of each polymorph found in the shells of different species.
TiO2 is one of the most researched materials in surface science because of its low toxicity, availability¸ and extensive range of applications, such as self-cleaning windows, catalysis, and photovoltaics.
Due to this prevalence across the field, TiO2 is commonly known as the prototypical metal oxide surface. Many polymorphs of TiO2 are known to exist, but only two occur naturally in abundance – Anatase and Rutile. While Rutile-TiO2 is the more thermodynamically stable form, Anatase-TiO2 is more energetically favorable when forming nanoparticles at atmospheric pressure and temperature – conditions which are used when growing films by solution based processes in solar cells or other similar devices.
In addition, Anatase-TiO2 has previously been known as more photocatalytically active when compared to Rutile-TiO2, but new research suggests that the greatest photovoltaic efficiencies are obtained in devices that include a mixture of both polymorphs.
Experiment and Results
XPS spectra across a wide range of binding energies, called survey spectra, were acquired from natural crystals of Calcite and Aragonite (Figure 2), in order to establish the elemental composition. As predicted in the case of naturally occurring crystals, surface contamination was seen in the form of sodium and silicon. Next, high resolution spectra of each photoemission peak were acquired to facilitate the determination of chemical states present at the surface. The presence of aliphatic carbon contamination, besides the carbonate peak, is indicated by the C 1s photoemission peak. Aliphatic carbon contamination is removed from the surface through repeated cycles of sputtering with Ar1000+ clusters at 6kV produced by the Thermo Scientific™ MAGCIS™ (Monatomic and Gas Cluster Ion Source), and reacquisition of the survey spectra confirms the removal 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 observed from the overlay of the survey and valence band spectra acquired from the cluster-cleaned crystals of calcite and aragonite (Figure 4), the two different polymorphs of CaCO3 could not be distinguished with the XPS technique. However, the combination of XPS and cleaning with argon ion gas clusters helps to determine that the surface has the right stoichiometry and is free of contamination, thus enabling Raman spectra to be obtained with absolute confidence in the chemical and elemental composition of the region of interest.
The Raman spectra acquired from the two different crystalline forms of CaCO3 are shown in Figure 5. The higher shifted peaks correspond to what are known as internal modes that are associated with the carbonate anion. Although a slight shift can be seen in some of these peaks, they are similar in both polymorphs. The peaks seen in the lower shifted region are the result of lattice modes and rely on the way the carbonate anions are arranged in relationship to each other in the crystal structures. In addition, the lower symmetry in the Aragonite structure results in many more peaks in this region of the Raman spectrum. The actual assignments of these various vibrational modes have been addressed elsewhere.3
Raman spectroscopy can be used to determine the ratio of polymorphs in mixed samples. By using polymorphically pure samples of titanium dioxide (TiO2) as references, a method can be developed to determine the percentage of each polymorph present. Coincident XPS-Raman was used to analyze a total of five powders: pure Rutile-TiO2, Pure Anatase-TiO2, and three mixed powders.
High resolution C 1s, O 1s, Ti 2p, valence band photoemission spectra, and survey spectra were repeatedly obtained from all the TiO2 powders, while performing cleaning cycles with Ar2000+ gas clusters at 4 kV to eliminate surface carbon contamination. With the use of argon ion gas clusters, surface contamination is removed without affecting the chemistry of the underlying substrate, as shown by the lack of a metallized titanium peak at 455 eV; this occurs when etching with monatomic argon because of the preferential sputtering of oxygen, forming sub-stoichiometric TiO2-x.
Figure 4. XPS spectral overlay (cluster-cleaned)
||Aragonite as received
||Calcite as received
||Aragonite cluster cleaned
||Calcite cluster cleaned
Figure 5. Overlay of the Raman spectra obtained from the CaCO3 crystals.
Subtle differences are seen in the valence band spectral shapes obtained from the pure Rutile-TiO2 and Anatase-TiO2 powders using the XPS method (Figure 6), but these differences are slight, making it difficult to determine the relative proportion of each polymorph difficult (Figure 7). On the other hand, the visibly different Raman shift peak positions seen in the pure Rutile-TiO2 and Anatase-TiO2 powder spectra make it easy to distinguish between the Rutile and Anatase polymorphs (Figure 8). In addition, the spectral profiles can be employed as references for non-linear least squares fitting of the Raman spectra obtained from the mixed powders (Figure 9), allowing simple and rapid determination of the Anatase:Rutile ratio of the three mixed powder samples. The 142 cm-1 peak in the Raman spectrum of Anatase can also be used as the basis of a quantitative method to determine the Anatase percentage in a mixture of Rutile and Anatase. These types of quantitative methods can be easily developed with the Thermo Scientific™ TQ Analyst™ software.
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.
||Anatase : Rutile Ratio
Figure 9. Comparison of the Raman spectra of the mixed TiO2 powders
XPS is used to quantitatively determine the chemical and elemental composition of any solid material that is compatible with ultra-high vacuum. Raman spectroscopy, on the other hand, is used to identify referenced compounds through careful spectral matching using spectral databases and searching algorithms. The XPS-Raman combination on a single instrument, iXR and Nexsa, enables more powerful analysis of a material than either of the technique in isolation, with the purity, cleanliness, and stoichiometry of a sample established using the XPS technique. This combination also enables the identification and quantification of molecular structures to be established using Raman spectroscopy.
Since both XPS and Raman spectroscopy are aligned to the same position within the vacuum system, all the time-intensive aspects of locating the same analysis point when transferring from instrument to the next is eliminated, providing absolute confidence that the information obtained has all come from the same sample region, which is especially useful when investigating non-uniform samples.
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, Mollusc 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 – Materials & Structural Analysis.
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