Particle size effect of hydrogen-induced lattice expansion of palladium nanoclusters

In situ synchrotron x-ray diffraction experiments on bare palladium nanoclusters prepared by inert-gas aggregation and size selected (1.7–6.0 nm) show significant changes in lattice parameter upon hydrogen loading and a narrowing of the miscibility gap, as the cluster size decreases. The results show that the miscibility gap is open for all cluster sizes studied, in contrast to previous literature results from surfactant-encapsulated palladium clusters. We interpret these results by showing that the nature of the surface is critical in the hydrogenation behavior of the nanoclusters.

Figure 1

(Color online) X-ray diffraction patterns recorded at 0 and 50 Torr hydrogen pressures for the 6.1 nm diameter cluster sample. The peaks have been indexed to the fcc structure.

Figure 2

(Color online) (a) Raw data (black points) and fits (red/gray thick lines) for 6.1 nm cluster (111) peak at pressures corresponding to the $\alpha$ phase only (0 Torr), $\beta$ phase only (50 Torr), and in the miscibility gap (15 Torr). The 15 Torr fit is comprised of two peaks, as shown (green/light gray thin lines). The positions corresponding to the two peaks are shown by the vertical black lines. (b) Raw data (black points) and fits (red/gray thick lines) for the (111) peak at pressures within the miscibility gap. Only the 6.1 nm data can be fitted with two peaks (green/light gray thin lines). The vertical lines show the position of these two peaks.

Figure 3

FWHMs and lattice parameters for (a) 6.1, (b) 3.0, and (c) 1.7 nm clusters upon increasing (left) and decreasing (right) the hydrogen pressure.

Figure 4

(Color online) Lattice parameters obtained from fitting the peak positions from x-ray diffraction patterns for the three cluster diameters studied as a function of hydrogen pressure. The arrows indicate the direction of the hydrogen pressure change.

Figure 5

(Color online) Lattice parameters corresponding to ${\alpha }_{\text{max}}$ and ${\beta }_{\text{min}}$ for the three samples under study, compared with bulk and nanograined Pd (Ref. 11) and clusters with surfactant shells (Ref. 12). The solid and dotted trend lines are given as guides.