Silica (SiO2) is one of the most important oxides in advanced technology and catalysis. The preparation of crystalline thin silica films on a metal substrate, first reported by Schroeder et al. [1], was further developed in the recent years [2-3]. The experimental (scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS)) and theoretical (DFT) studies [2] showed that the ultrathin silica film grown on Mo(112) consists of a two-dimensional network of corner-sharing [SiO4] tetrahedra, in contrast to the isolated [SiO4] cluster model previously proposed by Chen et al. [3].
In this work, we provide further evidence for the network model employing photoelectron spectroscopy (PES) with synchrotron radiation and ultraviolet spectroscopy (UPS). In particular, angular resolved PES studies clearly showed two different O species which can be unambiguously assigned to the Si-O-Si and Si-O-Mo bonds in the film. Our further XPS and IRAS studies corroborated by DFT calculations show that, depending on the preparation conditions, the film may additionally contain oxygen atoms bonded only to the Mo atoms, referred to as an "O-rich" film. We have studied interaction of the metal atoms such as Pd and Au with the "O-poor" and "O-rich" silica films by STM, XPS, IRAS and adsorption of CO as a probe molecule. The results show stronger interaction of Pd with "O-poor" films as compared to the "O-rich" films and than Au on both films. These results are consistent with the theoretical predictions of Giordano et al. [4] who predicted that, in contrast to Au, the Pd atoms can penetrate through the ultra-thin silica film and bind strongly at the SiO2/Mo interface.
In order to study the effect of the film thickness on the metal interaction with silica films, we have developed preparation of multilayer silica films on Mo(112). The structure and the interaction of the metal with these "thick" films will be discussed in comparison with the ultra-thin film.
References:
[1] T.Schroeder et al., Surf. Rev. Lett., 7 (2000), 7.
[2] J.Weissenrieder et al. Phys. Rev. Lett., 95 (2005), 76103.
[3] M.Chen et al., Phys. Rev. B 69 (2004), 155404.
[4] L.Giordano et al. J. Chem Phys., 124 (2006), 34701. |