Surface enhancement factor

From the mean crystalline diameter of ca 4 pm and the diameter of a C.00 molecule (ca 1 nm), one can estimate that only a fraction of ca 0.0015 of the (Y>o molecules is located at the surface of the particles. Assuming that effective polarization transfer only occurs for C60 molecules located at the surface, one concludes that the observed signal enhancement of 15 5% corresponds to a polarization enhancement factor of ca 100 30. However, it should be noted that this simple estimation neglects the influence of spin diffusion which can lead to a transport of spin polarization into the bulk, resulting in a lower surface enhancement factor. 

The reaction channels were coated by the sol-gel technique, revealing a 2-3 pm thick alumina layer with pore diameters ranging between 1 and 5 nm having a maximum at 4 nm. Owing to the preparation technique, no surface area data could be obtained but a surface enhancement factor of 430 m2 rrf3 was determined. Pd was introduced as active component by wet impregnation. Subsequently, the catalyst was activated at 80 °C and reduced in H2. 

As diseussed in Referenee 1, it is well documented that, with our standard nanostmetured support partieles, the baseline pure Pt catalyst has a mass specific surface area of about 7 m /g. For our standard loadings of 0.22 mg/cm, this corresponds to an effective surfaee roughness factor (or SEF, surface enhancement factor) of 12-15 cm /cm. Figure 2 shows duplicate measurements (Series A, Series B) of the SEF for the PtAB-2 ternary catalyst at three different loadings eompared to the standard Pt at 0.22 mg/em. It is seen that the ternary has the same surface area with half the amount of Pt as the standard pure Pt catalyst. This doubling of the mass... 

The effective increases in the Raman scattering cross sections, or surface enhancement factors (SEE), are typically about lO fold on electrochemically roughened silver, gold, and copper. Few works have been reported on the other free electron metals such as alkali metals. Many transition metals have relatively weak surface enhancements. Their SEE values depend on the nature of the metal, for example, 10 10" for Pt, Rh, Ni, Co, and Fe, and 10 for Pd. 

The influence of nanorod aspect ratio on the SERS surface enhancement factor (SEF) was systematically investigated by Murphy and was shown to be a significant parameter for the case of isolated nanorods 23). Therefore, the SEF dependence on nanorod length for the Ag nanorod array substrates was probed. Figure 2 highhghts the results of this study, while details can be found elsewhere (28,33). It is concluded that optimum SERS substrates are those with nanorod lengths of 900 mn which provide a SEF of 5 x lO for BPE (33). 

Figure 2. Surface enhancement factors plotted as function of nanorod length from 190-2000 nm. Each data point represents the average SEF taken from 5 different locations and the error bars represent the standard deviation. (Adapted with permission from (33)).

Zeman, E.J. and Schatz, G.C. (1987) An accurate electromagnetic theory study of surface enhancement factors for silver, gold, copper, lithium, sodium, aluminum, gaUium, indium, zinc, and cadmium. The Journal of Physical Chemistry, 91, 634—643. 

Types of packings Surface area a, m /m Void fraction e Surface enhancement factor Fffi ... 

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