Surface paramagnetic centers

In the case of surface centers, one deals with the PCs characterized by the considerably narrower distribution functions than those typical of the stabilized PCs in the bulk. The isotropic HFI constant between the unpaired electron and the central silicon atom is close to 480 G even in the case where the surface paramagnetic center forms due to the mechanical activation of silica at low (room) temperature, and the material is highly disordered. Although the EPR spectrum of these centers is well resolved, it is, nevertheless, broadened as compared with the spectra of the defects of similar structure stabilized at the surface of the RSi samples (prepared at 1300 K). 

Irradiation of high surface area silica has produced several well-defined paramagnetic centers, one of which appears to be an intrinsic defect in the... 

Paramagnetic Centers =Si-0" on the Silica Surface (Non-Bridging Oxygen Center) 275... 

MSi and RSi). By involving these centers in the reactions with gas-phase molecules, one can conduct directional chemical modification of their structure (Table 7.3) and quantitatively convert them from one form to the other. This lies at the basis of the method for preparing the surface-stabilized PCs of the type (=Si -0)2Sia -r, where r — H(D), OH(OD), NH2, CH3(CD3), C2H5, etc. in silica [18,19]. This method amounts to generating low-molecular mobile radicals r in the reaction between the surface-stabilized paramagnetic center S and the gas-phase r1— r molecule ... 

After the adsorption of inorganic (02, 03, NO, N02, S02, CO, C02, etc.) or organic molecules onto the semiconductor surface and especially after further illumination of a sample prepared, different stable or relatively stable radicals are easily recorded by the EPR method. Several important systems in which charge separation created organic radicals were described in detail in Chapter 1 of this book. Some additional information concerning adsorbed pentane, methane, ethylene, benzene, methylbenzenes and m-dinitrobenzene can be found in publications [41, 60, 69-74]. Further, we will shortly discuss some structural features of paramagnetic centers formed under chemical activation or irradiation of the adsorbed oxygen or NxOy molecules. 

The next step in approximation of the theory for EPR measurements was done by Atsarkin and co-workers in [138]. Basing on Fel dman s results, the researchers studied dipolar broadening and spin exchange narrowing of EPR lines from the paramagnetic centers distributed on the solid surface, and a simple formula for a signal of free precession S(t) was expressed [138] ... 

Additional information on the nature of copper complexes formed on the Ti02 surface and their relative amount has been obtained in [285]. Fig. 8.18 represents typical EPR spectra of Cu2+ ions adsorbed onto Ti02 nanoparticles at pH 3.1 and 6.0. At pH 3.1, the spectrum is typical of isolated mononuclear Cu(II) complexes. Some noticeable broadening of the ESR lines corresponding to the parallel orientation of Cu(II) complexes in the external magnetic field, which is observed even for the most magnetically diluted samples, is indicative of the existence of several paramagnetic centers with similar, but not identical structures. The increase of Cu(II) concentration results initially in an increase of... 

Saito, H., Nagashima, Y., Hyodo, T., Chang, T.B. (1995) Detection of paramagnetic centers on surfaces of amorphous-Si02 fine grains using posi-tronium , Mater. Sci. Forum 175-178, 769. 

While the effective g value is expressed in terms of three principal values directed along three axes or directions in a single crystal, only the principal values of g can be extracted from the powder spectrum rather than the principal directions of the tensor with respect to the molecular axes. (Therefore it is more correct to label the observed g values as gi, g2, g3 rather than g gyy, in a powder sample.) In the simplest case, an isotropic g tensor can be observed, such that all three principal axes of the paramagnetic center are identical (x = y = z and therefore gi= gi = g-i). In this case, only a single EPR line would be observed (in the absence of any hyperfine interaction). With the exception of certain point defects in oxides and the presence of signals from conduction electrons, such high symmetry cases are rarely encountered in studies of oxides and surfaces. 

Through the above series of examples, it is clear that EPR offers many advantages for the characterization of paramagnetic species on oxide surfaces. The obvious Umitation of the technique is of course that it only detects paramagnetic centers. However, if paramagnetic centers, such as defects, radicals or transition metal ions, are involved in a heterogeneous process, then EPR is the ideal spectroscopic technique. To date most of the studies applied to oxides have used the traditional cw-EPR method. Modern pulsed techniques offer far more sensitivity and resolution than cw-EPR, and it is certainly hoped that these pulsed techniques will be more widely used as commercial spectrometers become more numerous in research laboratories. Compared to cw-EPR, the numerous hyperfine techniques... 

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