X-ray Photoelectron Spectroscopy (XPS) Study of Layered Double Hydroxides with Different Exchangeable Anions

Layered double hydroxides (LDH) containing various exchangeable anions were studied to show how X-ray Photoelectron Spectroscopy (XPS) can provide information on the local environments of the different elements within the interlayer anionic groups and their possible influence on the LDH interlayer hydroxide surfaces. As such XPS can potentially provide additional information about these systems that cannot be obtained by other common spectroscopic methods such as infrared and Raman spectroscopy. A Mg6Al2X(OH)16. 4H2O with X representing interlayer anions CO32-, PO43-, SO42-, MoO42-, CrO43-, Fe(CN)64-, and Fe(CN)63- was studied. The hydroxide layer structure is characterized by the Mg 2p and Al 2p with a binding energy of around 50.1 and 74.5 eV for the normal CO32- containing LDH. The O 1s contained three peaks related to the layer OH-groups at 531.6 eV, interlayer CO32- at 530.5 eV and interlayer water at 532.4 eV. Similar observations were made for the other interlayer anions showing characteristic P 2p, S 2p, and Mo 3d peaks. Intercalation with CrO43- shows that a significant amount of the Cr6+ has been reduced to Cr3+. Finally, the intercalation of hexacyanoferrate in hydrotalcite showed the potential of XPS in detecting changes in the oxidation state of Fe upon intercalation in the LDH with a change in the Fe 2p peaks with a shift in binding energy and the possibility to determine the amount of reduction of Fe(III) to Fe(II). In general, the XPS high resolution scans of P 2p, S 2p, Mo 3d, and Cr 2p show that slightly lower binding energies are observed compared to the binding energy values for the corresponding anionic groups as part of a rigid crystal structure, such as in minerals. Overall, the influence of the nature of the interlayer anion on the binding energy of the elements (Mg, Al, O) in the layered double hydroxide structure is minimal and considered to be within the experimental error of XPS. Detailed analysis of XPS data in combination with infrared and Raman spectroscopy show how XPS can provide additional information not readily available via vibrational spec-troscopy.