Raman Shift (cm1)

Figure 14. Raman spectra of the [email protected] nanocapsules. After [125], P. Z. Si et al., J. Mater. Sci. 38, 689 (2003). © 2003, Kluwer Academic.

Ek = hv — Eb. The experimentally measured energies of the photoelectrons are given by Ek = hv — Eb — Ew, where Ew is the work function of the spectrometer. The electron binding energies are dependent on the chemical environment of the atom, making XPS useful to identify the oxidation state and ligands of an atom. XPS instruments consist of an X-ray source such as Al or Mg Ka, an energy analyzer for the photoelectrons, and an electron detector. The analysis and detection of photoelectrons require that the sample be placed in a high-vacuum chamber. Since the pho-toelectron energy depends on X-ray energy, the excitation source must be monochromatic. The energy of the photo-electrons is analyzed by an electrostatic analyzer, and the photoelectrons are detected by an electron multiplier tube or a multichannel detector such as a microchannel plate. Surface spectroscopy serves to get information about the chemical nature of sample surfaces or surface near regions. XPS allows one to analyze the elemental surface composition quantitatively. Different binding states of the detected elements may be distinguished. Angle resolved XPS is a nondestructive method to investigate the distribution of elements or functional groups in the depth of the sample surface. The knowledge of the chemical surface composition and the kind of functional surface group is fundamental to evaluate surface reactivity and apply chemical and physico-chemical methods to modify the solid surface.

XPS is a very sensitive surface analysis technique providing elemental quantification and chemical information of the nanocapsules. The XPS detection therefore provides the information on the electrons and thus the materials the electrons belong to. Figure 15 shows XPS spectra of Fe(C) nanocapsules produced in methane at 13.3 kPa [113]. Figure 16 represents Fe2p3/2, Co2p3/2, and O1s XPS spectra of as-prepared Fe-Co(C) nanocapsules and their surface cleaned samples [120]. XPS provides direct evidence for the existence of elements in the shells of the nanocapsules, and also for the formation of the core/shell structures. The binding energies of electrons can indicate well whether the atoms are free metal or oxides or ceremics, from which one can


0 0

Post a comment