Electronic d-band properties of gold nanoclusters grown on amorphous carbon

The electronic d-band properties are important factors for the emerging catalytic activity of Au nanoclusters of sub-5-nm size. We analyzed the d-band properties of Au nanoclusters grown on amorphous carbon supports by photoelectron spectroscopy using synchrotron-radiation light coupled with high-resolution ion scattering spectrometry which enables us to estimate the size and shape of Au nanoclusters. The d-band width (W-d), d-band center position (E-d), and apparent 5d(3/2)-d(5/2) spin-orbit splitting (E-SO) were determined as a function of a number of Au atoms per cluster (n(A)) and an average coordination number (n(C)) in a wide range (11 < n(A) < 1600). The W-d and E-SO values decrease steeply with decreasing n(A) below similar to 150 owing to band narrowing which is caused by hybridization of fewer wave functions of the valence electrons. However, E-d shifts to the higher binding energy side with decreasing cluster size. The rapid movement of E-d is attributed to the dynamic final-state effect, which results in higher binding energy shifts of core and valence states due to a positive hole created after photoelectron emission. We have estimated the contribution from the final-state effect and derived the approximated initial-state spectra. Modified data, however, still show a slight movement of the d-band center away from the Fermi level (E-F) although the E-d values for Au nanoclusters are closer to E-F compared to the bulk value. This behavior is ascribed to the contraction of average Au-Au bond length with decreasing cluster size.