Mossbauer spectroscopy fine particles

McCammon et al. have studied fine nickel particles using Ni Mossbauer spectroscopy [22]. The measured average hyperfine field of 10 nm particles at 4.2 K was 7.7 T for nickel foil, it was found to be 7.5 T. Application of an external magnetic field of 6 T caused a reduction of the hyperfine splitting to 1.5 T as a consequence of the negative hyperfine field at Ni nuclei. 

The other approach is to study real catalysts by using in-situ techniques such as infrared and Mossbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) and XRD, either under reaction conditions, or - as occurs more often -under a controlled environment after quenching the reaction. These in-situ techniques, however, are usually not sufficiently specific to yield the desired atom-byatom characterization of the surface, and often they determine the overall properties of the particles. The situation is represented schematically in Figure 1.8. 

Abstract. A chemical composition and structural parameters of specially prepared catalyst for the pyrolytic synthesis of carbon nanomaterials have been studied by X-ray diffraction, Mossbauer spectroscopy and electron microscopy. A plenty of chemical transformations in the catalyst have been monitored. The inert (Mgi xFxO) and active, very fine particles of the catalyst (MgFe204) components which are involved in the process of carbon nanofibers were revealed. 

Experimental determinations of for fine particles show some inconsistencies. This is not surprising, as detailed studies on ultrathin films have shown that changes may depend on several parameters. Generally, is lowered with respect to the saturation magnetization value for the corresponding bulk material. The decrease should be mainly due to spin canting at the surface, that is, disorientations of spins directions, as evidenced by Mossbauer spectroscopy experiments. " A decrease of the magnetic moment per spin at the surface is also possible for metallic particles. However, for the present it is difficult to predict such effects because of the lack of systematic studies on the different types of particles. 

Similar to bulk materials, through the recoilless fi-action and static hyperfine interactions, Mossbauer spectroscopy can be sensitive to a wide range of phenomena relevant to fine-particle systems. The main differences with respect to bulk studies come from the specific properties of the atoms near the surface. 

B. SURFACE HYPERFINE FIELD. The demagnetizing field and the Lorentz field are not defined for the atoms at the surface. Calculations of the magnetic dipole fields at atoms near the surface in fine particles and thin films of a-Fe show that only the first surface layer is perturbed, with variations of the order of lOkOe depending on the position at the surface. Studies of thin and ultrathin metal films by Mossbauer spectroscopy (see, e.g.. Refs. 31, 224, 238, and 239) show that one or two atomic layers are perturbed, as for the magnetization (see Section B.3). The surface hyperfine field may be larger or smaller than in the bulk at... 

As already discussed, the size, the shape and the dispersion are characteristic parameters of metal particles and will determine their properties. It is possible to characterize them as follows (i) The size of the metal particles (transmission electron microscopy, (TEM) [79-81], extended X-ray absorption fine structure (EXAFS) [82, 83]), (ii) their structures (X-ray diffraction (XRD), TEM), (iii) their chemical composition (TEM-EDX, elemental analysis), (iv) the chemical state of the surface and bulk metal atoms (X-ray photoelectron spectroscopy (XPS)), Mossbauer [84], thermo-programmed-reduction (TPR)), (v) chemisorption capacity toward probe molecules such as H2, O2, NO and CO (volumetric or dynamic measurements) [85-90]. 

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