Experimental and computational studies of silicon-oxygen bonding structures on metal surfaces
A different approach to modeling metal-metal oxide chemistry has been attempted. Specifically, the chemistry of Pd, Pt and silica is of concern. The chemistry of Si, O and metal atoms has been studied through the deposition of thin layers of Si and O on the order of one monolayer thick onto Pd(111) and Pt(111). Studies were done through a combination of experimental and computational efforts.
Si chemistry on Pd(111) was studied through the adsorption and reaction of SiH4. Dissociative adsorption to a SiHX surface species was observed through high resolution electron energy loss spectroscopy (HREELS), occurring at 200 K. Complete dissociation occurs above 250 K. Pd-silicide formation is suggested above 500 K.
Si-O bonding on Pd(111) was studied through the adsorption and reaction of SiH4 and O2. The Si-O bonding structure formed on Pd(111) revealed through HREEL spectra indicates a silica-like structure that is thermally stable. The structure measured by HREELS after high temperature treatment appeared to be largely independent of beginning Si and O coverages. Auger electron spectroscopy (AES) results also show evidence of Si-O bonding consistent with a Si suboxide. Temperature programmed desorption (TPD) studies indicate the structure formed by high temperature treatment seemed to act only as a site blocker to H2 and CO. However, structures observed prior to high temperature treatment showed a higher H2 desorption temperature than the clean Pd(111) surface.
Comparisons of SiH4 and O2 adsorption and reaction chemistry on Pt(111) to the same chemistry on Pd(111) revealed a greatly different result. No silica-like layers were indicated, however, Si-O bond vibrations were present in HREEL spectra. Furthermore, no perturbed H2 TPD spectra were observed.
However, density functional theory (DFT) results reveal Si-O species possessing vibration modes similar to those observed on Pt(111). Similarities between the results on Pt(111) and the low coverage structures simulated using DFT suggest higher Si and O coverages were achieved on Pd(111). DFT results also showed a Si-O bonding structure continuous across the simulated metal surfaces. This structure consisted of Si atoms bound to the metal surface with O atoms bridging the Si atoms.