D-band shifts

This expression indicates that the change in hybridization energy is opposite and proportional to the shift of the d band center. Thus, if the d band shifts upwards the hybridization energy increases and vice versa. Strain and the associated shift of the d band can be brought about by growing the desired metal pseudomorfically on another material with a different lattice constant. The term pseudomorfic means that the overlayer grows with the same lattice constant as the substrate. The overlayer may thereby be strained or compressed depending on the lattice constants of the two materials. 

Figure 6.33. Trends in reactivity for an overlayer deposited pseudomorfically on a substrate. The diagonal gives the position of the center of the d band for the pure metals. The other numbers indicate the shift of the d band by formation of a pseudomorfic overlayer, irrespective ofw/hether it can be realized. Notice that in the low/er left-hand corner the d bands shift upw/ards, leading to higher...

The same theory, i.e. Eqs. (86) and (87), allows us to understand why CO and similar molecules adsorb so much more strongly on under-coordinated sites, such as steps and defects on surfaces. Since the surface atoms on these sites are missing neighbors they have less overlap and their d band wUl be narrower. Consequently, the d band shifts upwards, leading to a stronger bonding. 

Again the d band centers are found to describe changes in adsorption energies quite well [34-37]. This is illustrated in Figure 4.12, through the electrochemically determined variations in the hydrogen adsorption energy, for different Pd overlayers as a function of the calculated d band shifts [38]. 

As described in Chapter 2, a number of spectroscopic surface methods give information relating to d band shifts [45]. Ross, Markovic and coworkers have developed synchrotron-based high resolution photoemission spectroscopy to directly measure d band centers giving results in good agreement with the DFT calculations [46]. Another possibility is to exploit the fact that in some cases a shift in the d states can be measured as a core-level shift, as the d states and the core levels shift... 

Figure 4.16 also shows that there are additional effects due to direct interactions between adsorbates that are not described using the d band model. A large adsorbate like S, will have a sizable overlap to the valence orbitals of the incoming molecule, giving rise to a repulsion which is larger than what can be readily explained by the indirect interaction through d band shifts. 

Figure 11.5. (a) Raman D band shift versus strain for PVA/PVP/SDS/SWNT composite films containing 5wt% of nanotubes, (b) Tensile load curves for the same composite film (upper curve), compared to pure PVA or PVA/SDS/PVP films. Reprinted with permission from reference (22). 

To quantily the metal dissolution trends, and to offer comparisons of the stability of surface Pt atoms in different environments, we reported the development and application of a computational approach based on first-principles calculations on metal slabs, using the methodologies explained in this chapter. The method allows us to evaluate the electrochemical potential shift AU (V) for the dissolution of Pt atoms in an alloy surface, relative to the potential at which the same reaction would take place on pure Pt(lll) surfaces. Recent investigations in our lab have found interesting correlations between the potential shift for the onset of surface oxidation of Pt in Pt-based alloys with respect to the same potential in pure Pt surfaces and the d-band shift of the surface atoms, reflecting the changes in the electronic structure due to alloying. The results will be published elsewhere. 

This finding, together with the XRD data for the nanocrystal core properties [58], demonstrated that both the core and the surface of the bimetallic nanopartides exhibit bimetallic alloy properties. The detection of both Au-atop and Pt-atop CO bands on the surface of the alloy nanoparticles of a wide range of bimetallic composition can be correlated with the electronic effect as a result of the d-band shift of Pt in the bimetallic nanocrystals. There exists a stronger electron donation to the CO band by a Pt-atop site surrounded by Au atoms in the bimetallic alloy surface than that from the monometallic Pt surface as a consequence of the upshift in d-band center of Pt atoms surrounded by Au atoms, which explains the preference of Pt-atop CO over the Au-atop CO adsorption. The observed decrease of the Pt-atop CO band frequency with increasing Au concentration is in agreement with the d-band theory for the bimetallic system [172]. 

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