Paper NS+BI-MoM2
Formation of Metallic Nanoclusters on TiO₂ by Deposition and Sputtering

Monday, November 9, 2009, 8:40 am, Room L

The discovery that Au nanoclusters are catalytically active when supported in an oxide matrix¹ has led to an abundance of surface studies, particularly those concentrating on metal nanoclusters deposited on single crystal TiO₂. For example, STM showed that Au clusters grow on TiO₂(110) without a wetting layer and that the size of the clusters increases as more metal is deposited.² We previously used low energy alkali ion scattering to interrogate such clusters grown by evaporation, measuring the neutral fraction of the backscattered ions with time-of-flight spectroscopy.³ We were able to show that the neutralization depends on the electronic structure of the clusters, and therefore changes as a function of cluster size. Our group also demonstrated a novel method for the production of nanoclusters by Ar⁺ sputtering a thin Au film deposited on TiO₂.⁴ As this initial work was performed with Au, it is not clear whether nanoclusters displaying quantum size effects could be fabricated by sputtering other materials. In the work presented here, Ag and Pt are evaporated onto TiO₂(110) and thin films of these metals are sputtered. The materials are then interrogated via the scattering of 2.5 keV Na⁺. It was found that while both Pt and Ag form nanoclusters by evaporation, only Pt forms nanoclusters after sputtering. This may be due to differences in the surface free energy of the metals, or of the propensity of Ag to oxidize. Future efforts will include temperature programmed desorption to test the chemical activity of the metal nanoclusters, as well as the use of other substrates.

¹M. Haruta, et al., J. Catal. 144, 175 (1993).

²M. Valden, X. Lai and D.W. Goodman, Science 281, 1647 (1998).

³G.F. Liu, Z. Sroubek and J.A. Yarmoff, Phys. Rev. Lett. 92, 216801 (2004).

⁴P. Karmakar, G.F. Liu, Z. Sroubek and J.A. Yarmoff, Phys. Rev. Lett. 98, 215502 (2007).