(760c) Experimental and Computational Studies of Si-O Bonding Structures Formed by Submonolayer Coverages of Si and O on Metal Surfaces

The bonding that occurs between Si, O and Pd atoms at metal-insulator interfaces is important in applications such as heterogeneous catalysis and catalytic metal insulator semiconductor hydrogen sensors. A better understanding of this chemistry may provide information regarding metal-support interactions at an atomistic level. Using both surface science techniques and computational chemistry techniques, the adsorption and decomposition of silane (SiH₄) on clean Pd(111), O- covered Pd(111) and O- covered Pt(111) surfaces has been studied. Recent work has focused on the Si-O bonding structures formed on Pt(111), and using density functional theory (DFT) to elucidate the observed differences between Si-O structures formed on Pt(111) and Pd(111). The use of small molecules to deposit Si and O on single crystal surfaces results in a relatively well defined model interface, which can be used to study relevant chemistry.

We have studied the thermal chemistry of SiH₄ on the clean and O-covered Pd(111) surface and on the O-covered Pt(111) surface. The reaction between adsorbed silane and oxygen on both Pd(111) and Pt(111) resulted in SiO_X surface species that differs between the Pd and Pt surfaces. These differences are revealed primarily through high resolution electron energy loss spectroscopy (HREELS), but differences are also observed via auger electron spectroscopy (AES). High temperature annealing of SiO_X covered Pd(111) surface resulted in long range order of the suboxide through the presence of phonon modes. These modes are absent in the structures formed on Pt(111), and in fact the highest coverage structures on Pt(111) lead to observations of a structurally different Si-O bonding structure than what is seen on Pd(111). AES results show low coverages of Si on the Pt(111) surface, and show Si atoms heavily influenced by the metal surface. DFT modeling of small discreet Si-O bonding structures in addition to one continuous Si-O bonding structure show more similarities in calculated vibration modes to experimental derived spectra on Pt(111) than Pd(111). Additionally, the modeled structures are much more stable on a Pt(111) slab than on the Pd(111) slab. DFT results showed the modeled continuous Si-O bonding structure to be markedly more stable than other modeled structures. The suboxide covered surfaces can be used as model interfaces which can serve as platforms to study the interfacial chemistry of the simple but important probe molecules, H₂ and CO. In this presentation, our progress in using a combination of experimental and theoretical tools to analyze this model interfacial chemistry, and its dependence on temperature, metal composition, and other conditions will be discussed.