Size-dependent lattice structure of palladium studied by x-ray absorption spectroscopy

X-ray absorption spectroscopy measurements have been made on 7-, 12-, and $23\text{−}\mathrm{nm}$ palladium particles, along with bulk material as reference. With decreasing particle size, the extended x-ray absorption fine spectra reveal a substantial contraction of the nearest-neighbor distance and a reduction of the average coordination number, but an increasing Debye-Waller factor. Meanwhile, the x-ray absorption near edge spectra show a decreasing density of unoccupied states with decreasing particle size.

Figure 1

(Color online) XRD spectra of bulk and 7-, 12- and $23\text{−}\mathrm{nm}$ Pd particles at $300\mathrm{K}$.

Figure 2

TEM images of (a) $7\text{−}\mathrm{nm}$, (b) $12\text{−}\mathrm{nm}$, and (c) $23\text{−}\mathrm{nm}$ Pd particles.

Figure 3

(Color online) Normalized $K$-edge absorption coefficients of Pd samples at $300\mathrm{K}$. The energy at the edge inflection point is $24350\mathrm{eV}$.

Figure 4

(a) Weighted EXAFS spectra as $k$-dependence of $k^1\chi \left(k\right)$. (b) Fourier-transformed $k^1$-weighted EXAFS spectra $(\Delta k=3.23–10.86{\mathrm{\AA }}^{-1})$.

Figure 5

(Color online) (a) Nearest-neighbor distance contraction versus the inverse of Pd particle size. (b) Average coordination number versus Pd particle size. (c) Debye-Waller factor versus Pd particle size.

Figure 6

(Color online) $K$-edge XANES spectra of Pd samples at $300\mathrm{K}$.