Surface shift

Figure 6.16. Illustration of the d-band model governing surface chemical bonding on transition metal surfaces. As the d-band center of a catalytic surface shifts downward more antibonding orbitals become occupied and the surface bond energy of an adsorbate (here an oxygen atom) decreases. An upward shift in the d-band center predicts strengthening of the surface bond.

As mentioned earlier, the existence of surface shifted core levels has been questioned.6 Calculated results for TiC(lOO) using the full potential linearized augmented plane wave method (FLAPW) predicted6 no surface core level shift in the C Is level but a surface shift of about +0.05 eV for the Tis levels. The absence of a shift in the C Is level was attributed to a similar electrostatic potential for the surface and bulk atoms in TiC. The same result was predicted for TiN because its ionicity is close to that of TiC. This cast doubts on earlier interpretations of the surface states observed on the (100) surface of TiN and ZrN which were thought to be Tamm states (see references given in Reference 4), i.e. states pulled out of the bulk band by a shift in the surface layer potential. High resolution core level studies could possibly resolve this issue, since the presence of surface shifted C Is and N Is levels could imply an overall electrostatic shift in the surface potential, as suggested for the formation of the surface states. 

Experimental surface shifts in metal and nonmetal levels... 

Demonstration of surface shifts in the nonmetal levels, i.e. in the C Is and N Is levels, was not feasible in these first efforts. These levels are located considerably deeper in energy and were inaccessible. Recent developments in synchroton radiation instrumentation,15 however, allow high resolution core level studies to be performed involving these levels.16-20 The first such study was carried out on the (100) and (111) surfaces of VC16 and a surface shifted C Is level was clearly observed (Figure 25.2). A C Is... 

The message conveyed in this section is that surface shifted core levels... 

Table 25.1 Surface shifts determined experimentally and estimated using the thermochemical model...

The uncertainty in the experimental value is typically 0.02 eV. a No surface shifted level could be resolved in the core level spectrum. 

In this model the surface shift is expressed as a sum of partial shifts with the dominant contribution being the partial shift originating from the loss of coordination at the surface. This shift is given as the product of an effective concentration parameter, cv ( Sl), and a difference in cohesive energies. 

For the nonmetal level the sign of the surface shift is correctly predicted... 

Figure 25.8 (a) C Is spectrum recorded from the clean WC(0001) surface and after an oxygen exposure of 30 L. (b) W 4f7/2 core level spectra (dots) recorded, using a photon energy of 100 eV, from the clean WC(0001) surface and after oxygen exposures of 30 L, 100 L and 300 L. The solid lines through the data points show the results of a curve fitting procedure. The curves below each spectrum show the bulk (B), surface shifted (S) and chemically shifted... 

Surface shifts in core level energies of transition... 

This chapter summarizes recent experimental findings on transition metal carbides and nitrides and shows, in section 25.2, that measurable surface shifts do exist both in the metal and nonmetal levels. A few application examples are presented in section 25.3. These experimentally determined surface shifts may stimulate new theoretical efforts to explain their existence and lead to a better understanding of the surface electronic properties. To date, no ab initio calculations of surface shifts for transition metal nitrides and carbides, including final state relaxation effects, have... 

For WC,19 ZrN20 and NbN,20 on the other hand, surface shifted components were revealed in both the metal and nonmetal levels. For WC(0001) a surface component shifted to smaller binding energy was clearly observed both in the C Is and W 4f spectra (Figure 25.4) with surface shifts of —0.66 eV and —0.38 eV, respectively. Both the WC(0001) and VC(lll) surfaces were expected to be metal terminated so the observation... 

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