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 ($E_{\text{SO}}$) 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 $E_{\text{SO}}$ values decrease steeply with decreasing $n_A$ below $~$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.

Figure 1

Au nanoclusters valence-band spectra analysis. (a) Measured spectrum of 0.3-ML Au deposited on $a$-C/Si(001). The spectrum was smoothed and the Shirley background was subtracted. (b) Background-corrected spectrum of Au(0.3ML)/$a$-C/Si(001) and a carbon substrate contribution. (c) The difference spectrum representing Au nanoclusters’ valence band. The d-band parameters $W_d$, $E_{\text{SO}}$, and $E_d$ of Au nanoclusters are shown. (d) The derivative of the (c) spectrum and smoothed curve are used to find the points of inflections as boundaries of the $d$ band.

Figure 2

(a) MEIS spectrum observed for 120-keV He${}^{+}$ ions incident on Au(0.05 ML)/$a$-C/NaCl. Incident and detection angles are $-{45}^{°}$ and ${45}^{°}$ with respect to the surface normal. The best-fit (red curve) was obtained assuming a bimodal size distribution with the $G_S$ group: $d=0.9$ nm, $h=0.45$ nm, $\sigma =25$%; and $G_L$: $d=3.0$ nm, $h=1.5$ nm, $\sigma =25$%. Volume and areal occupation ratios ($G_L/G_S$) are 0.3 and 0.09, respectively. (b) TEM image for Au(0.05 ML)/$a$-C. (c) A histogram of diameter distributions obtained by MEIS (green) and TEM (orange).

Figure 3

(a) The valence-band spectra of Au nanoclusters (measured at $h\nu =60$ eV) at different coverage (after a substrate contribution subtraction). The dashed line guides the eye to indicate the shifts of $d_{3/2}$ and $d_{5/2}$ peaks to higher binding energies. The bulk polycrystal Au valence band is also shown for the reference. (b) The Au $4f$ core-level spectra of Au nanoclusters (measured at $h\nu =140$ eV) at different coverages. The dashed line guides the eye, showing the shift of Au $4f_{5/2}$ and $4f_{7/2}$ lines to the higher binding energies. The bulk polycrystal Au core-level spectrum is also shown for the reference. (c) Derived position of the $d$-band center and shift of the Au $4f_{7/2}$ core line depending on the cluster size.

Figure 4

(a) An example of fitting of the convoluted Fermi-edge spectrum of the bulk Au to the valence-band spectrum of Au (0.6 ML) nanoclusters ($\tau =0.5$ ps, $R_{\text{ave}}=0.85$ nm, $\sigma =0.25$). (b) Deconvoluted valence-band spectra of Au nanoclusters.

Figure 5

(a) The $d$-band center derived from the deconvoluted spectra of Au nanoclusters. (b) The comparison between the $d$-band width, $W_d$, measured before (open symbols) and after (filled circles) deconvolution of the spectra. (c) The comparison between the apparent $d$-band spin-orbit splitting $E_{\text{SO}}$ measured before (open symbols) and after (filled circles) deconvolution of the spectra. The $n_A$ values is given in logarithmic scale for the latter two plots to improve data readability.

Figure 6

(a) The $d$-band width ($W_d$) and (b) the apparent $d$-band spin-orbit splitting ($E_{\text{SO}}$) depending on the average coordination number of Au atoms in gold nanoclusters. The open circles represent the original measured data, and filled circles correspond to the data after deconvolution treatment.