Dynamic structure in supported Pt nanoclusters: Real-time density functional theory and x-ray spectroscopy simulations

The nature of local atomic and electronic structure at the nanoscale is of both fundamental and technological importance. For example, supported metal nanoclusters exhibit a number of unusual phenomena including large structural disorder and bond-length contraction with increasing temperature. We investigate this behavior for a prototypical ten atom Pt cluster supported on $\gamma$ alumina using temperature-dependent, real-time simulations based on density functional theory/molecular-dynamics and x-ray spectroscopy theory. The simulations reveal a complex dynamical structure on multiple-time scales including librational motion of the center of mass and fluctuating bonding characteristics, which explain many of the unusual properties.

Figure 1

(Color) Time-elapsed rendering of the structure of a ${\text{Pt}}_{10}$ cluster on [110] $\gamma {\text{-Al}}_2{\text{O}}_3$. The purple and gold spheres represent Pt atoms that are metallic and oxidized, respectively, with the latter being bound to surface O atoms, while the red and turquoise spheres represent oxygen and aluminum atoms, respectively. The “blurriness” of a given atom characterizes its range of motion.

Figure 2

(Color) Pt-Pt pair distribution function and fits (dashed lines) to asymmetric 1nn distributions $g\left(r\right)$ generated by ad hoc Morse potentials (see text). The inset shows the effective Pt-Pt pair potentials that characterize the thermal and structural disorders at 165 (blue) and 573 K (red).

Figure 3

(Color) Comparison of theoretical (bottom curves) and experimental (top curves shifted for clarity) x-ray absorption near-edge structure (XANES). The theoretical spectra are obtained from a configurational average of 32 random conformations extracted from the last 5.5 ps of the MD simulation. The error bars on the theoretical calculations indicate the standard deviation of fluctuations between the various configurations.